First, the iMac is not powerful, I think you're mistaken. It's using a laptop graphics card and for what it's worth everyone agrees that it performs really well (Gee, even guys from Penny Arcade play WoW on the Intel iMacs and say it's great... :-\ and those are some serious gamers). You wouldn't get any better from a PC running Win XP with same specs (the 512MB RAM doesn't help gaming much either).
Second, sometimes it's really better to set your facts straight regarding things—the integrated graphics card is way better than the old Radeon 9200 (or whatever it was on PPC Minis). Sure, it's not great in some things like UT 2004, but it's way better at handling video playback, Core Image and some other games. In fact, unlike the PPC Mini, the Intel one at least fully supports all of Tiger's effects.