For music related stuff, you'll need a massive hard drive. Otherwise all I can say is a good processor and a decent amount of ram. I have a dual core at 2.9ghz and 4gb of ram, and I've had absolutely no problems running fruity loops etc. However, the one problem I do have is that I've only got a 300gb hard drive and I have around 10gb free. Samples take up a lot space, seeing as with games and stuff I'd never ran over 100gb of space.
My friend picked up a laptop for €350 second hand with 8gb of ram and an i3, and he uses his laptop for music. Albeit a small screen, he's had no problems and only uses his laptop for music. So $500 is definitely possible. Look on ebay and amazon and stuff.
Also you're US so try newegg. Low delivery costs.