Shouldn't have issues with Windows 7 since that's widely been adopted. There is a nearly MANDATORY update to Service Pack 2 for Vista since all my copies already have Service Pack 1, as you know, Vista was basically vilified for it's technical failings and equated to the worst MS OS since Windows ME, I've used and continue to use, both WinME and Vista and didn't face those issues, but SP1 for Vista is only a partial solution if you use that OS, make sure to get SP2 installed. XP using SP3 is only supported early into next year, then Microsoft will discontinue any patches and updates. You can game and use XP offline till the end of time if you wish, but just be aware, if you live connected to the Web, don't rely on XP since there will be no way to stop future malware!!
I would say, have more then one machine capable of playing it, but also be advised from a lower spec standpoint, I have tried both PC variants and Episodes from Liberty City is actually MORE demanding then GTA IV for PC's initial release.
My game crashed to Blue Screen of Death while initiated the first opening mission, this was deemed caused by a lack of power, so I upgraded power supplies, but on your end, I'd say that graphics card could easily stand to be replaced, even by a more robust older gen card like the 8800 Ultra from nVidia.
Been doing some reading in advance of a GPU upgrade, I read that GDDR5 is a fair improvement over all previous memory types, so much so that it allows a smaller memory interface to be more efficient then in years past, say, starting out, memory interfaces would be 64bit, 128bit, or 256bit on up.
Good cards supporting DirectX 10 and Shader Model 4 on up will load and should play GTA on PC just fine, but my card also is a base model, the HD5450 ATI, but it can still manage to load the game, but with some difficulty animating it fast.
The memory of frame buffer is only on a 64bit interface, but it's not GDDR5. For nVidia, you should go after a more expensive Kemplar or Kemplar 2 that have done very well so far, so much so that nVidia plans to stick with it's current cards for all of next year I believe. Considering the cost, up around $200USD, it might be worth it this time to make the investment in a current mid to top end card