The step from Sims to Sims 2 was pretty much amazing, with the extreme graphics revamp etc.
I stopped my Sims 2 collection after I got Sims 2 Nightlife on release date and it fucked up my computer, idk why, but I've got my laptop now. I still don't know why I didn't carry on collecting the expansion packs.
But Sims 3 looks AMAZING, the fact that you can go next door at any time is pretty cool!