Also, if the Dell breaks, expect to pay DOUBLE for the replacement part from Dell...
I have let my feelings be known on this issue. If you want a serious rig, build it yourself.
If you want INSANE performance, it will cost you the price of a Hyundai to get it. $500 will not even get you a video card. Sorry.
I have even seen gaming rigs costing around $12000, but you get a nice widescreen monitor to go with it.
May I suggest a site for you to look at:
http://www.cyberpowersystem.com/Default.asp
No proprietary stuff inside, which means you can upgrade with whatever you like!
If you want to put it all in a new case anyways, then you may as well build it yourself.
WARNING! If you go the ridiculous route, be warned of heating issues with smaller towers. The best gaming rigs come in BIG full tower cases.
Just for ghits and shiggles, I will try and configure you the best PC for $500 or less at Cyberpower.
EDIT*
If you have $600, then I advise EVERYONE try to get this. It is a mistake on Cyberpowers site. They have a $1000 processor for almost NOTHING!
I doubt you can get away with it, but who knows. It is worth a shot!
http://www.cyberpowersystem.com/system/cfgpd.asp?v=d#configurator_top
Click on the Extreme Edition 840 CPU. THis should EASILY make this system DOUBLE the price quoted!
If you don't believe me:
http://www.newegg.com/Product/Product.asp?Item=N82E16819116214