With your budget you can get some pretty good stuff. I saw your build in the previous thread and you can keep some stuff like your graphics card and sound card as both of those look like some very good equipment.
For your CPU, I suggest why not go with the latest Ivy Bridge chip which will handle pretty much anything you can throw at it: https://www.newegg.com/Product/Product.aspx?Item=N82E16819116506
That chip is an LGA 1155 socket, which means you need a motherboard with the same socket. This ASUS motherboard will give you USB 3.0 and SATA 6 support. USB 3.0, if you don’t know, is the latest USB standard that’s a lot faster than 2.0. SATA 6GB/s is also a lot faster than SATA 3GB/s.
https://www.newegg.com/Product/Product.aspx?Item=N82E16813131781
That’s a standard ATX board size, so it’s fairly large. It also supports up to 32GB of RAM, so you’ll never run short of memory. In order to support the Ivy Bridge chipset you’ll just need to download the latest BIOS for the board.
Here’s where you get that: https://usa.asus.com/Motherboards/Intel_Socket_1155/P8Z68V_LX/#overview
OK, so far you’re running $200 + $124 = $324. RAM is fortunately very, very cheap. Whenever you check for RAM you should just read the compatibility list provided by the motherboard manufacturer. You can find that at the link above. Let’s go with 8GB of RAM so you can handle any game and have a lot of multitasking capability.
https://www.newegg.com/Product/Product.aspx?Item=N82E16820231402&Tpk=F3-10666CL7D-8GBRH%28XMP%29
Those two RAM chips give you 8GB (2x4GB) and they’re fully compatible with the motherboard I picked out.
Total, you’re now running $200 + $125 + $50 = $375.
For all of this I do highly suggest a new power supply. Something a little beefier than 550 watts, and simply newer so it has no wear-and-tear. I suggest Corsair, and modular so you can save a lot of wire clutter. Modular just means that aside from the main power cords, you can select which cords you want to plug in and which you don’t. The pictures show it all. Also, this is an extremely efficient power supply and is 650W.
https://www.newegg.com/Product/Product.aspx?Item=N82E16817139035
Well, that’s $150 so now you’re up to $525. Still within budget.
All of these components are fully compatible and will give you some awesome computing power. Maybe you’d like a new computer case while you’re at it?
I highly suggest this Antec 300 Illusion case: https://www.newegg.com/Product/Product.aspx?Item=N82E16811129066
I’ve used it before, it has tons of cooling fans (a must), plenty of room and cable management. No frills but I think it looks good, not to mention is REALLY simple to clean.
So that case goes for $70, which brings your total up to $595! How about that!