I'd get an ATI 5850 or similar. Those new Nvidia chips are just too hot and power hungry for the performance advantage.
And as for the GDDR5, it is part of the gfx card and not the mobo. The mobo has zilch contact with the graphics ram. Everything going to that ram via the mobo goes through the GPU so the mobo's speed (or lack there of) will have no effect.
Neb speaks truth!