If you are buying a new processor, get the AM2 socket model. Else you are back where you are at now with no upgrade path. Putting out a choice there would be close to your pick now:
PICK#1 4 IDE devices, 4 USB ports, less cost ASUS M2N-E SLI Socket AM2 NVIDIA nForce 500
PICK#2 2ide devices, 6 USB ports, better Raid controllor, more cost ASUS M2N-SLI Deluxe Socket AM2 NVIDIA nForce 570 SLI
For the difference in price I would go with the 7900, they will keeep up the X1950 PRO for some games and not much different in others.
RAM depends on the vendor you buy from. I ampreety sure AMD runs best with DDr2-800 RAM
Your other parts should work with these parts.