For your situation, upgrading this system would be beyond a waste. It would be like ubber waste.
I know you are probably bored and want to tinker...here is what you should do.
1. Go buy an aftermarket CPU cooler, this one to be specific - http://www.canadacomputers.com/product_info.php?cPath=8_129&item_id=035257
2. put the cooler on and overclock your CPU to about 3.8 ghz by increasing the multiplyer and adding a little bit of voltage to the CPU (~100mv). You can go to 4 ghz but it may become unstable. You have a "Black Edition" with unlocked multiplier...use it.
3. Get 2-4 gigs of ram and a 64 bit OS
4. Get a solid state drive for your OS
5. in a year or when more demanding games come out (Battlefield 3) buy a new power supply and add another 5850 in crossfire.
Doing any other CPU or GPU upgrades is just foolish... you're set for at least 3 more years with the upgrades above.
