All following is based on my personal experience, so your milage may vary
Unless you go with an 64 bit Win OS, you will be limited to something between 3 and 3.5 GB seen by the OS. More limiting is the 2 GB address space per program, limit wihich you do not have with MacOSX, a.f.a.i.k.
But if that means changing licences, it becomes a different story.
@ work i have been very pleased with HP (xw series) , very unhappy with Dell (optiplex). ok, ok, comparing apples and oranges...
i do prefer ati graphic cards as their drivers are more stable
if i am not mistaken, i read that future development of adobe CS4 would use the rendering capabilites of the GPU for speeding up things...
Nicolas