They did some extra work yeah, and for 2008 it was impressiveish, but all they really did was add in some support for PC features and smashed the resolution up. The decent rig comment I wouldn't agree with, though, you need a lot better than what a game of GTA's quality
should take to run it without stutters. My rig for example is a 6850 1GB w/ FX-4100 @ 3.6Ghz, from what I've read my hardware is all in harmony with one another (lol) and push on another perfectly, so I'm running a pretty solid 1GB rig all around, the king of thing you'd of had to pay over £1,000 for in 2008. Infact the best card in 08 was a Nvidia 512MB, so to get the kind of performance we all get now would have been next to impossible in the year it was actually released.
The games unoptimized to ****, and is one of the worst ports going, but the game's core mechanics are so ****ing good, that I don't care, especially now AMD have fixed Rockstar's **** ups, and improved overall performance tenfold.