Build it yourself. Cheaper with higher quality parts, and it’s much, much easier than you would think. Plus you will know how your computer works so you can fix it if you ever need to.
Basically, I would recommend a Core 2 duo, like this one. Includes a heatsink and fan, you might want to upgrade the stock fan if you’re doing serious overclocking, which I doubt. $110
https://www.newegg.com/Product/Product.aspx?Item=N82E16819115206&Tpk=core%202%20duo%20e7400
Get a good, at least 500w power supply like this one. $90
https://www.newegg.com/Product/Product.aspx?Item=N82E16817139004
This case is simple, cheap, and high-quality. Very good reputation. Includes 2 fans, you can add more if needed (again, only really if you’re doing serious overclocking). $60
https://www.newegg.com/Product/Product.aspx?Item=N82E16811129042
An ATI radeon 4670 will play any source game at >60 fps assuming your processor is good enough, I get about 60 and my processor sucks. You can upgrade this if you want to, your mobo will probably have 2-3 pci-e slots on it so you can get more than one. A 4670 is kind of mid-range, there are other people here who can give you more information if you need it. Basically it’s just a lot of bang for your buck. If you want to spend a little bit more, 2 4850s in crossfire (basically the 2+ cards are connected as one) have a good price/performance ratio.
https://www.newegg.com/Product/Product.aspx?Item=N82E16814102820 - 4670 - $65
https://www.newegg.com/Product/Product.aspx?Item=N82E16814150337 - 4850 - $110 (each)
RAM you want probably 4 gigs, especially if you get Vista (nowhere near as bad as people say, loads of people here use it) and you can probably get a free upgrade to windows 7 with it. $52
This 4gb kit is pretty decent:
https://www.newegg.com/Product/Product.aspx?Item=N82E16820231122
Harddrive is really up to you how much space you need, unless you download lots of porn, games, or have a crapload of flac music 500gb is probably all you need to start. HDD upgrades are incredibly easy to do yourself. SATA is newer & faster than IDE.
something like this is fine. $65
https://www.newegg.com/Product/Product.aspx?Item=N82E16822136319
Disk drive, a lot of people only get one, I like having two to copy discs, but oh well. Cheap and easy to upgrade. Even easier than harddrives, because you don’t have to format them first (windows install does this for you). Again, SATA. $27
https://www.newegg.com/Product/Product.aspx?Item=N82E16827151187
And the mobo - ties everything together. Depends on your cpu and to an extent ram, this one works with the examples I gave. $95
https://www.newegg.com/Product/Product.aspx?Item=N82E16813157143
and cables for the hdd/dvd drive(s) - make sure they’re the right kind for the given drives. $2 each
https://www.newegg.com/Product/Product.aspx?Item=N82E16812123111
If you go the crossfire route with the gpus, then you need a crossfire bridge. this one’s good. $7
https://www.newegg.com/Product/Product.aspx?Item=N82E16814998002
hope I haven’t forgotten anything, this would be a fully functioning comp if you bought the parts specified. Much cheaper and higher quality than anything made for you, unless you paid tons of $$ for it.
I assume you already have a monitor/mouse/keyboard, if not you can choose those on your own.
The cost for it (this is subject to change, you may find that it goes down) is roughly $730 US, with the 2 4850s in crossfire and one hdd, one dvd drive. Not bad for a box that’ll last you for a good while. And someone’s going to come along and pick this apart with links for a much better configuration.
I’ve edited this a lot in the 15 min since I first posted, just fyi.