Since i built my computer, i have a problem with some weird graphic effects. Here`s an example with GTA San Andreas.
Look at the neck of T.J. It look likje a shading problem.
The same occur with Company of Heroes, Crysis and some more games. I have tried with different drivers with no luck.
Did San Andreas look better on your old computer? I would bet that you simply have the graphics maxed out and weren't expecting crappy console shading. Keep in mind GTA SA was made for old lo-res consoles not high res computers