Dude your mo-bo is old and it does not have a PCIE 2.0 card. That's a big problem and you will have to upgrade your BIOS in order to make a 2.0 card work.
I would suggest the following.
1. Upgrade your BIOS to the latest version. If the last update is not from mid 2009 you have a problem.
2. Pick up a 5670 or a 5750. Anything else will be severely bottle necked. I have a Athlon 5600+ and anything above these cards is a waste of money. Frame rates are way to choppy.
3. What power supply do you have? If you don't have a proper supply the 5670 is the only option you have.
4. Increase your RAM. Its too less and I hope you have it set up for dual channel.
5. Maybe a couple of months down the line your can upgrade your processor. Even the X6 works on your board. More than 2 cores are a must for smooth game play. Jumping from a 5600+ to a phenom II was a big improvement for me.
Let me know if you need anymore advice. Our rigs are relics. And I know how hard it is to find real info.