Question about video cards, motherboards, and video ram (ddr2 vs ddr)

Do mobos that support DDR VIDEO ram also support DDR2 VIDEO ram without a problem? 

As in if I trade out my ATI Radeon 9550 with 256MB DDR for a PNY GeForce 7600GS with 512MB DDR2 am I going to have problems about what "speed" of video ram the mobo supports?  Both cards are AGP 8x, which is what my mobo supports.

Any PCI-E card?

I was wondering why an 8800 GTX has no list of motherboards that are compatibile with it, but the 7950 GX2 does

it doesnt matter its the card that supports the ram not the mobo. As long as that mobo is PCI-E it will support any PCI-E card. Its the system ram that the baords needs to supprot new boards support ddr2 ram and are imcompatable wiht DDR ram.ForlornHope

This is what I figured.  Thanks, guys.