The game always runs fine, until the city gets big, but I ran a big city with my ATI card, then as soon as I put the 7950 in, I tried the same city out, and it ran fine. It's got a lot to do with cpu, too. Sim City 3 and 4 ran kinda garbagish on pretty much any system, along with the sims... they just don't seem to be written well at all, fun, but not written well.
The Sims 1, on my grannies computer, still runs like crap, with a gig of RAM, and an AthlonXP 2500, with a 6200 series Nvidia. I've seen the sims run, very slowly, on a 133Mhz pentium, with 32mb of RAM, it still ran though... Puttin it on such a superior machine such as grannies should have made it run flawlessly, but it seems like, since those types of games rely so heavily on scripts, you need dual quad cores just to run them smoothly. One reason it runs like crap on grannies computer is that it has ALL the expansion packs on it, and about a million houses and families (she loves the sims). Any time you buy one of these maxis games, you may as well accept the fact that they usually run poorly. The last maxis game I owned that ran smoothly was SimCity2000, haha.
It's mostly cpu for SimCity4 though, having a Nvidia card helps though, I guess, It helped me out a lot. Google "simcity 4 choppy on ATI" and see what you find....