My GF and I could never get Sims 2 running well, even on fairly modern dual-core machines (although she does seem to insist on using Sims 2 to build skyscrapers) - in contrast our experience of Sims 3 was that it ran better than Sims 2 on the same PC.
I've tried running Sims 3 on a single core 3200 (stock) with 8600GT and 1.5GB ram, and it wasn't much fun. It really does want a dual core CPU. Ram makes a major difference too. . . 1.5 was not really enough. It's okay if you're in one place, but the way Sims 3 seamlessly moves between locations really stuggles without fast everything.