Need help finishing gaming build

Hey everyone! I need some help finishing my PC gaming build. Iv'e been stuck with a regular home PC for quite some time now and a few months ago I decided to start upgrading it.

I first bought an HD Radeon 5850 graphics card that came with a Thermaltake TR2 600w power supply. I had hoped (foolishly) that I could stick those parts in my home PC and it would accept the upgrade. I was wrong, it didn't work as I expected. So now I'm trying to find a good CPU, motherboard, and ram to fully finish the upgrade. I don't need to worry about any other components as my old PC has a good case, hard drive, os, monitor... all that jazz.

My budget is approximately 400-500 dollars for just the CPU/mobo/ram.

APPROXIMATE PURCHASE DATE: In two weeks

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: hardcore gamin', surfing the web.

PARTS NOT REQUIRED: power supply, graphics card, case, hard drive, disc drive, OS, mouse, keyboard, monitor

PREFERRED WEBSITE(S) FOR PARTS: Newegg but I'm open to anything

COUNTRY OF ORIGIN: USA

PARTS PREFERENCES: AMD, quad or hex core, 4GB of ram

OVERCLOCKING: I will overclock the CPU

SLI OR CROSSFIRE: I can see myself buying another 5850 down the road so the mobo should be crossfire compatible.

MONITOR RESOLUTION: 1680 by 1050 for now but I probably will upgrade to 1920 by 1080 eventually

ADDITIONAL COMMENTS: I'm building this to max out all games on the market right now (especially dx11 games) like Bad Company 2, Stalker: Call of Pripyat, Crysis, Supreme Commander 2, Starcraft 2, WoW, Fallout 3... you get the idea. I also have Windows 7 64 bit

I'm really interested in hex core processing because I feel it will help keep my computer from becoming obsolete down the road. Although I know quad core processors are still running strong. Should I go with hex or quad core?

Just some idle questions here. Is a Thermaltake TR2 600w power supply going to be compatible with whatever CPU/mobo you guys choose? I can get a new power supply if I need to but I'd prefer not to. Also, is 4GB worth of ram fine for gaming? I'm fairly sure it is but I want to hear it from somebody with more experience.