Heyyo,
I reccomend an ATI Radeon 9600 Pro, or a nivida fx5700 (NOT LE or anyother kindof suffix they suck). You can find them at decent prices of about $130 cad. I run the game with a 9600 pro and I can get some pretty good graphics. Las Venturas really beats down any computer since it's such a flashy town (lol, VRock Hotel pwns).
Now, I have an odd graphical problem where I have to leave the special effects on high. What happens is if I don't it starts having loading problems and I get a lot of black textures at a distance.. really annoying.