Which mac is the best solution to work with Cubase SX and many VST instruments (Spectrasonics)? G4 dual, G5 perhaps? Wich soundcard can I use for a G5?

SX doesn't work all that well on ANY Macs (or PCs, for that matter) - check out the steinberg forums and listen to all the Mac users grumbling about a long, long list of outstanding bugs that version 2 hasn't entirely wiped out. It's a good program, but a lot flakier than Logic.

Dual G5 would be the ultimate Mac. But is SX even dual-processor aware? I don't think version 1 was. By the same token, is SX now G5 ready? Answer both those questions and you'll have your answer.