PCI-Express x16 is for the GPU, so the X1900XT Slots nicely in there.
get the Opteron 165, they OC to similiar OC'd speeds as the 170.
dont get SLI or crossfire, UNless you want to run at resolutions like 1900x1600, which i doubt.
SLI (Nvidia) and Crossfire (ATI) Are where you run 2 GPU's in parrallel giving you "Double"
So just stick with a Single X1900XT and you'll be a happy camper.
Next get a 2x 1GB Kit of RAM. Like G.skill DDR500.
Ultra-D by DFI is fantastic, tho i havent got to try mine out yet cos the rest of my components havent arrived yet due to crap shipping.
Aint that right PZERO
So anyway, thats the basic breakdown of what you should get.