First thing’s first, I third the motion for newegg. Even if you don’t decide to buy from them, they’re a great resource for researching parts and reading reviews. They’re the first place I look to now when buying computer parts. Generally, they’re pretty cheap, too, and they ship pretty fast, as long as you don’t choose the free shipping option (not speaking from personal experience though).
Anyway, just thought I’d share a few tips that I was told when I first started out building my own PC after a Power Supply failure that fried everything (curse you, eMachines!)
1. Choose your processor (CPU) and your motherboard together. The biggest decision you’ll probably make is choosing a good processor/motherboard combo, since you’ll be suck with those pieces of hardware longest after buying them, whereas with just about everything else, you can swap out and upgrade as you please, later on. Since they go hand in hand, Don’t ultimately decide on one until you’ve decided on the other as well. It’s good to choose a processor first, and then look for motherboards that support that processor, but if the motherboards that support it don’t have the features you want, or if they’re just too expensive, it might be prudent to just look at motherboards for a while, forgetting about the processor you initially picked. Once you find a few motherboards that you think you might like, see what kind of processors they support, and go on from there, until you’ve found the pair that you want.
2. Features to keep in mind when buying a motherboard: What kind of CPU socket, how many hard drives it supports (ATA or SATA), how much RAM it supports, what kind of RAM it supports (DDR, DDR2, DDR3, etc… do yourself a favor: do not get DDR) how many RAM slots are available, how many PCI-e slots are available (used for graphics cards, mostly), how many PCI slots are available (for network/audio/TV tuner/etc. cards), and what kind of on-board video/audio it has. It’s not really necessary to get a board with an amazing on-board video chip, since you should be buying a dedicated video card anyway (especially since this is a gaming rig) but it’s a good thing to take note of, and might actually come in handy if your dedicated graphics card (or even just your graphics card cable) ever craps out on you in the distant future.
Also, since this is a gaming rig, you’ll need to decide now on whether or not you want more than one graphics card. If you want to link two cards together (called “SLI” for nVidia cards, and I forget what AMD/ATI calls their linking technology…) you’ll need a motherboard with at least two PCI-e slots. Even if you don’t want to, or even if it doesn’t fit the budget right now, it might be something to keep in mind for the future. Otherwise, if you plan on buying a new motherboard before you buy a second graphics card, you’ll only need one PCI-e slot, and boards with only one PCI-e slot can sometimes be a tad cheaper.
3. When buying a graphics card, money is really the only issue. People will most likely be arguing for/against nVidia vs. AMD/ATI until the end of time. But in both cases, more $$$ = more power, and buying tomorrow’s technology today can sometimes get insanely expensive. Don’t be afraid to buy yesterday’s technology if you have to, to stay in your budget — doing this can actually save you a bundle — just decide on a price you think you’re willing to spend, and then find a card around that price that seems to offer a good amount of power for the cash you’re spending. Then, just make sure to read the reviews, maybe google for screenshots of games that were taken using the card you’re thinking about buying, search for things like “nvidia card x vs ati” or “ati card y vs nvidia” to see how well it stands up to the competition, that sort of thing.
Something to consider. Both Nintendo and Microsoft used ATI chips for the GameCube, the Wii, and the Xbox 360. PlayStation 2 used some chip designed specifically by Sony, but it does look similar to nVidia technology (IMHO), and PlayStation 3 actually uses an nVidia chip. I’m not saying which one is better, because if you spend enough cash on either, both can look amazing, but I do think it’s interesting to at least see the technology in action. I have no clue if the benchmarks prove this, (depends more on the card than the brand, I think) but generally, I’ve heard that ATI is better at doing things like anti-aliasing, whereas nVidia’s strength lies in rendering more polygons. I would have gone for an ATI, personally, had it not been for the fact that nVidia cards had better support in Linux, though I’m not sure if this is still the case anymore, since I haven’t worked in Linux for a good while now.
4. Get the Power Supply Unit (PSU) last, after you’ve decided on everything else. The reason for this, is so you can be sure to get one that will provide enough power (as well as the right connections) to all of the other hardware you’ve selected to purchase. There are calculators available online (like here and here) that make it fairly easy to determine roughly the amount of wattage you’ll need. You just select what hardware you have (or would like to have… remember, keep the future in mind! Better to get a more powerful supply than you need, than to under-power your rig) and see roughly how much wattage will be required. Anywhere from 400 to 500 watts should do fine for a budget gaming rig, I think. Then again, my budget gaming PC is about two years old, so things may have changed, you might need more or less. Just make sure to leave yourself plenty of room for upgrading and/or adding hardware in the future, but at the same time, don’t go overboard and (for example) get a power supply that powers more hard drives (or any other part) than you can fit on your motherboard or in your case.
Those are the four biggest decisions you’ll probably make when building your computer: CPU, Motherboard, Graphics, and Power Supply.
After that, (and these are fairly minor decisions… just read reviews and make up your own mind, find stuff that fits the budget, etc.!) you’ll want a [url=‘https://www.newegg.com/Store/Category.aspx?Category=15&name=Hard-Drives’]hard drive/url, a CD/DVD-ROM drive(s), a network card (unless your motherboard has one built-in), possibly a dedicated audio card (although people can argue for/against the need (or lack thereof) for dedicated audio cards forever… I personally don’t have one, my on-board audio is just fine for me), fans, and a case with good airflow that fits everything. Make sure the case you get fits the form-factor of your motherboard. (MicroATX is probably the most common, it’s about the size of your average Dell tower. Standard ATX is a bit large, better for home servers/closet computers than for desktop/gaming machines IMHO, but you do get more PCI slots that way. It’s up to you.)
That’s really all there is to it. Select your stuff, order it, open it, throw it all in the case, plug in the power/ribbon/SATA cables, twist a bunch of screws, power it up, install your operating system, choose a desktop wallpaper, fire up Steam… you’re good to go. Hope at least some of that helps. Good luck! 
You may even be able to squeeze in another 5770 if you play your cards right.
