The problem with $500 is that you need a MINIMUM expense for even a basic desktop. You can build an okay system for $500 but an extra $100 or $200 makes a HUGE difference in performance.
I personally recommend spending a little bit more than $500 and then upgrade from the 9800 graphics card later on.
1) 1156 Motherboard ($150)http://www.ncix.com/products/index.php?sku=43123&vpn=P7P55D&manufacture=ASUS
2) i5-750 CPU ($235)
3) 4GB DDR3 ($130)
4) WD Black 640GB ($75)
5) 600W PSU ($70, read reviews. I prefer more expensive ones.)
6) Antec 300 case ($60 on sale)
7) *You OS should be Windows 7 64bit. I assume you have that or Vista 64bit.
So to get down towards $500 we'd have to get an $80 motherboard and a much less expensive CPU which in turn would not you you much upgradeability in the future.
An extra $200 gives you a really great gaming computer (even with "only" the 9800) which in the future would be really awesome with something like an HD5870 1GB when prices drop.
Of course I don't know exactly what you want, but most people I talk to regret going too cheap and then they upgrade anyway and don't save money in the long run.