i can't speak for the Nvidia 6800.... i don't think it'll give you any huge advantage over the 9600 (unless, of course, you have that monster 30" LCD or work in 3D). Plus it eats up two PCI slots for it's monsterous self. I don't think it would leverage anything for Motion...

Shake gets the best performance with high speed media and cache drives more than anything else - it only uses OpenGL for the interface and all those noodles. I used Shake heavily for 2 years, but honestly, end of day, it was too specialized, and sometimes just a little frustrating (like you have 15 rotomattes you need to shift 5 frames... ugh, what a nightmare.)

I found 4 Gigs of RAM to really improve my workflow... bouncing from Combustion to Final Cut or Cleaner, etc... less time using virtual memory. High Def really like that extra RAM.