Well if you just wanna run windows, I would recommend one of the new AMD X2-3600 chips, they're very nice indeed and can be overclocked too (of course you don't have to).
Well, you know there are a LOT of variables to take into account when you buy a new PC. Of course everything is customizable. So you have to choose a motherboard, CPU, RAM, Optical drive (not a biggie

), Hard drive (cheap nowadays, isn't everything?), Case, PSU, blah blah blah.
The case I recommend is a if you can get an antec NSK3300, nice case, takes MATX motherboards. It includes a PSU and a quiet fan. Then get a nice mATX mobo for AM2, and an AMD X2-3600. Take it from there. Then see how much ram and hdd you can afford. I really digged this case since I read a review of it on silentpcreview.com
Do you need to buy a monitor as well with that $500?
These are just my initial thoughts. If you wanna run OSX rather go for an intel based system.
Then again you can try look for a Mac mini core duo that's on sale