Will this graphics card work in my rig?

You will probably want to OC that CPU. If not it will hold back the HD5850 in the more cpu intensive games. It should easily get up to around 3.3ghz on the stock cooler and then it should be fine.
What exactly is the native resolution of your monitor?

^+1

Proper answers from maziar.
To assess your performance improvement potential, run this test:

Run your games, but reduce the resolution and eye candy to a minimum. This will simulate what will happen if you upgrade to a stronger graphics card. If your FPS improves, it indicates that your cpu is capable of driving a stronger graphics card to higher levels of FPS.

I think your cpu will be OK as it is. To assess the capability of your cpu, run this test:

Keeping your graphics resolution and settings the same, reduce your cpu power. Do this by removing the overclock, or by using windows power management to set a maximum cpu% of perhaps 70%. If your FPS drops significantly, it indicates that your current cpu is a limiting factor, and that a faster cpu or overclocking would help.

None of those tests will tell him exactly what card is appropriate for or would be limited by his current processor/speed. It only tells him about his processing power in relation to his current card.
What game he chooses to use for the tests would also be extremely important.

Thanks for all your responses. What is the max price I should pay for a 5850 used? Or should I just buy a new card? Any recommendations, I don't care about going either Nvidia or ATI. I play all the games: Crysis,(hopefully Crysis 2) Fallout 3, Call of Duty, etc.

A new card would of course be preferable. What card you should get depends highly on the native resolution of your monitor and how much you are comfortable spending. With the HD5850 in particular an important factor in choosing your card is whether or not you plan on overclocking.

Several weeks ago I was overclocking my GFX and CPU, then my computer didn't post. I'm getting a new mobo. I think it was caused by CPU overclocking, but could it be caused by GFX overclocking. I used MSI Afterburner

Have you tried a different card in the system and/or the card in a different system?
Also have you tried resetting the BIOS to return it to default settings?

Yeah I tried both of those, neither worked. My replacement mobo is coming tomorrow so I'll post on the resaults.

Best answer selected by carfreak3504.