Welcome to anandtech, raven. Assuming you mean 450-500 dollars, not pesos or rupees, you can build a decent system these days with that kind of money, as long as you aren't wanting new hard drives, optical drive(s), case, etc. For ~$100, you can get either a Gigabyte GA-P35-DS3L or an Abit IP35-E motherboard. 8800GT's can be had for less than $200, not including mail in rebates. And you can get 2x2 GB of PC6400 RAM for less than $100, also. That would be ~$400, leaving you $100 for a processor.
edit: Those are newegg prices, BTW.