To clarify:
The number they like to list in MB specs is just for marketing. It's really a statement of how fast the front side bus will run. DDR2 800Mhz RAM will be fine, especially if it runs at 1.8V.
Any single PCI-E video card will run on any chipset, as long as the board has a PCI-E slot. If you wanted to add a second 4850 then you would want a P45, x38, or x48 chipset.