No, actually the PC (and xbox) versions of any of the GTAs came out later because Sony had an exclusivity clause with Rockstar as I understand it. That's why we had to wait a whole 'nother year for the games.
As far as a Vista only version, I think MS is really gonna push for it. I too don't see any use for Vista (yet) it's mostly a cosmetic upgrade. I guess has other cool interesting stuff, but not enough to warrant an upgrade (yet).
Regarding DX10, some developers say that there's not much of a difference between it and DX9, that most of the visual stuff is still doable with DX9. Also, I've read that these new consoles are not at a DX10 level better that DX9, but not DX10.
PC hardware to be needed: Considering the fact that these new consoles all have multiple core CPUs, If you're planning to upgrade, you better get at least a dual core CPU and DX9c class vid card.
But yes, we'll first need to see any images of the actual game..... drooling....... wiping.......drooling....