That depends. If X360 version comes to PC as a port there is a chance that it will be badly optimized. But GTAIV is pretty old version of RAGE engine. Newest is Max Payne 3 and we all know how well it runs on PC.
There's also a chance that GTAV will come out on next gen consoles, with somewhat improved graphics. If that happens I think that PC will get that version. It should be optimized because next gen consoles are pretty similar to PC.
The top quality probably won't look much better than on consoles.
Imho, it looks much better. Especially foliage rendering. Higher resolution textures of everything, better particle effects (richer), much better looking trees.