Looking at the screens for GTA:San Andreas looks like they're just releasing another "expansion pack". The graphics are pretty sub-standard now, and I'm sure the gameplay hasn't changed that much either.
The problem with the GTA:SA graphics is they are PS2 constrained. Although Rockstar get better and better at eeking the most out of the PS2 (and arguably only Gran Turismo 4 beats it in the scale / quality ratio) they are still running into laughably low limits, like 32Mb of RAM. Now, when they come to release a PC version, although they have some seriously powerful hardware (in comparison to PS2 or XBox) as their target, the game engine is designed for the PS2 and to rewrite it entirely is just not going to happen. That's why Vice City is a PS2 game but requires ridiculously high system specs in comparison to the original PS2. Admittedly the system requirements are not "ridiculous" now, but at release, they were quite steep.
However... the graphics are not really sub-standard by PS2 standards, remember they are 300x200 (approx - please correct) for a tv screen. At 1024x768, 1280x1024 etc like they'd be in the (as yet unannounced) PC release they'd look a heckuva lot better.
As for gameplay, horses for courses. No it won't have changed much. Refined, new features, new side attractions, but at the end of the day you'll be a guy that runs around a city doing missions and becoming the city bigshot criminal. If that ain't your bag, plenty more fish in the gaming sea
Personally, I love the GTA series