PSU & RAM: $73- Corsair CX500 & Corsair XMS 4GB(Note: PSU has $10 off with promo, also RAM is 4GB single kit to keep it under budget, in a month or so if you get spare $20 or so, add another 4GB kit)
Motherboard: $95- Asrock Z77 Pro3(Good motherboard from a reputable company, also if you get this mobo, do update the bios from the site)
GPU: $210- Zotac GTX 560Ti(Good GPU, capable of playing most games smoothly)
In terms of performance it's better than Radeon 6870, slightly slower than Radeon 6950. But makes up for it due to good overclocking nature
CPU: i5 3570K
MB: AsRock Z77 Pro3
RAM: G-Skill Sniper Low Voltage DDR3 1600 mHz (PC3 12800)
VGA: Sapphire Radeon 6850 1GB 256-Bit
CASE: COOLER MASTER Elite 430
PSU: OCZ ModXStream Pro 700W
SSD: Kingston V+200 120GB
OS: Windows 7 Home Premium 64 Bit
I believe I built a really good system for a very reasonable cost ($636). I did not buy a HD since I will use my old HD to store files and 120 GB SSD is much better to use for main applications, Windows and couple games I play. By the way for Windows 7 I bought: http://www.amazon.com/gp/product/B002MV2MG0/ref=oh_deta...
Which is $95, I divided it by three when I was adding it to the total cost since I am going to use it for three computers anyway. If you don't have more than one computer then you have to spend around $65-70 to buy a new WinOS.
By the way PSU, CPU, VGA and RAM listed above are very good selections for a budget computer with future OC or Crossfire options. But you should go to a MicroCenter to save $100 when you buy a CPU and MB. Otherwise the system I listed would have cost $700+.