And finally got a solution, but on my own. Got bored, so I did a clean OS X install to another drive and updated to 10.5.5. Spore worked fine. So, I tried yanking my second video card. Spore worked fine on the normal install. Put the video card back in, installed the 10.5.5 update again using the combo updater (not the delta one), and it works now.

Probably won't have time to get sucked into it till next weekend.