I think the games on the American PSN are straight ports, so it depends who's doing it. Could go either way.
GTA3 could potentially get an upgrade from the PS2 version purely because since that original version there's been the xbox and mobile versions, both of which included upgrades to poly count, textures etc. I'm not sure what upgrades VC has had.
I'm not sure I'd buy them - it'd depend on the price. I have all of the 3 trilogy on PC.