Unless you have pretty high specs (i5-3570k + GTX680) you won't get GT4 running too well. You need a really powerful computer to get it to run well. And with San Andreas, just get the PC version. It's better in every single way. Enthusia, I don't remember it ever running just right. Your best bet is softmod a PS2 or find a legit copy.
I probably should have put some asterisks in there. If you want to run it at higher than native res, you'll need a somewhat powerful computer. I know with my old AMD Phenom 9850BE and ATI HD4850 I couldn't run GT4 at an acceptable framerate even with native resolution. And now with my i5-3570k and GTX 680 I can run it 3-4x native res no problem.
No matter what I do PCSX2 won't use hardware acceleration for graphics on my machine so the highest framerate I can achieve is 10fps in jak&daxter
