The rule for gamers is anything lower than 9600XT is not worth buying but that was a year ago. So 9800 pro since that IS the gaming mainstream card now even in the eyes of ATI.
For Nvidia, its anything below 5900 Ultra is not worth mentioning but the games you listed are not really graphics heavy so a 9250 is good enough IMO. But if you want to play games like HL2, Doom 3, or Far Cry a 9800 Pro and the best bang for $$$ IMO 6600GT will perform great or good enough depending on the settings.
I was also surprise at The Sims 2 graphic requirements though my girlfriend plays that dumb game (we're married and all that in that game...scary!) and it barely run in her 9200 system. So she installed on my rig which has a 9800 Pro. So if you plan on buying/playing that game get a 9600XT or above.