That's odd. GTA3 worked fine on XP-sp2 & 3 for me (AMD 64 x2 Dual Core 3000+ and 9800GTX as graphics card.) That said, there other games I had that were not happy with XP or Dual Core Processors that were Windows based
I haven't tried them on 7-64 bit yet though. (I also have a different video card as that one went and died on me). I haven't got Vice City, but San Andreas was ok on XP.
I do have spares around so I may just chuck together one for older games (Say, a pure dos machine and a Win95 or 98 one just out or curiosity. I could use dosbox, but I guess it gets my old DX2-66 some more usage