Start with the following, do a search on newegg for them.
400-500W PSU (corsair, seasonic, PC-P&C, antec)
Cheap as hell case
Cheap as hell dual channel RAM (2 GB is fine)
Cheap as hell HDD. 1 TB drives are out at $65, that's about the best deal you can get.
Cheap as hell optical drive.
Cheap as hell Monitor of the size you want.
Once you have all these add up your total. If you include monitor, you're looking at around $250-$350 right there. That leaves you with $150-$250 for the 3 main components:
Motherboard - Anything works, some work better, but it's mostly moot. Look for what you want in terms of layout and audio.
CPU - You get what you pay for with severe diminishing returns. The cheapest CPU you can find is the best bang for your buck. AMD > Intel at low specs, and has a future, whereas Intel has no budget build with a future (you can run AMD's worst processor and best processor on the same socket). Dual core a must.
GFX - ATi is probably better here too, but I'd go for the smallest fab process. Any ATi card of *x00 where x is 8 or greater, and * is anything including nothing will run it fine. The 2-5000 series can probably run it fine at 6 or greater. Look at cost for a comparison. For nVidia, anything better than 8600 (includes 7800). That said, I'd probably go for the 4630 or something.
I can do this for ya, but please note the stuff you will be getting for free (case, monitor, optical drive, etc...).
"Parliamentary inquiry, Mr. Chairman - do we have to call the Gentleman a gentleman if he's not one?"