I think that's the smart thing to do, yes.
Your current MB supports Crossfire but HD 4850 would be handicapped in PCI-E 1 slots at x8. Compare that with a GA-X48-DS4 or aBit IX38 QuadGT where they would have PCI-E 2 slots at x16, i.e. 4 times more bandwidth available.
The PSU should be enough for HD 4850 Crossfire IMO.
Nice case. OK, I personally don't like lights and 80mm fans, but it's got good cooling. You need something very well cooled because the HD 4850 pumps out a lot of heat and the single-slot coolers with no vents don't let the heat get directly out. We've seen this before with the 8800GT. Maybe some brands will have dual-slot coolers, that would be great.
Memory: pick a motherboard first, then the RAM. Not all combinations work well, you know... There's some Corsair DDR2-800 2x2GB at newegg for only $80 now, for example.