Ok so first if the motherboard has a PCIE slot then it will run any card that goes in a PCIE slot. A card that is PCIE 2.0 will run in a 1.0 slot and a 1.0 card will also run in a 2.0 slot.
The memory dosent matter, System RAM dosent have to match graphics RAM. Your motherboard can only run DDR2, but your graphics card could be DDR2/GDDR2 GDDR3 or 4 or 5 it will still run.
Card wise, well what monitor size/ resolution are we talking here ? My personal preferance up to about a 22" screen would be a HD4850. While the 8800 series isnt a bad card they are getting on a bit now and the 4850 while about $30 more will be noticably better than the 8800 cards.