I would suggest hitting some PC tweeking forums. Those guys (I'm one of them) are alwasy upgradeing and sell stuff pretty cheap sometimes. Anandtech is a good spot too. I have built about 9 machines for friends in the last 2 years for about that price and nobody has complained yet. Little tricky with new stuff as it adds up quick.
but a couple recomendations.
Cant really beat AMD price wise right now, but even Intel has some sweeet deals.. keep in mind I would recomend Intel but when budget is considered its really hard to beat the value AMD has right now.
E5300 CPU is about $65 and OC's like mad!
mid range Video cards are all over for about $150
MB's that will OC a little and not break the bank are also under $100
memory is about $50 to $100 depending on how much ya want
that leaves HD/optical/case and PSU... I wouldnt skimp on PSU but a ok one shouldbe in the $50-75 range... all new stuff , kinda hard to do... but stick in a used part or two and you should be able to do it..
keep in mind when dealing with used stuff, Look for HEATWARE, dont deal with people who dont have it, read the HEATWARE to make sure they have been dealing lately and have good feedback. NO promises ever with used stuff, but you should be ok if you read and make sure you get some NO DOA promises..