This will work fine, if it's a 2002 model you'll want to make sure you get the QS'02 install CDs, or something newer/more universal. There's a 2003 eMac 9.2.2 CD on vtools that works fine on these, for example. (If you use older media like 9.2.1 from the QS01 or the digital audio, the install will work and the system will run but networking will not function.)
The second CPU will almost never do anything in OS 9, though. You could swap it for a faster single-core upgrade or just leave it in and accept that it's going to draw some power for no gain, unless you put OS X on it.
There's some MP-aware OS9 software. It's mostly some functions on high end pro software, with, like, iTunes and iMovie tossed in for good measure, if I remember correctly. Stuff like encodes or specific photoshop filters/effects and 3d renders are the things most likely to be MP-aware, but, you have to check with your specific application.
Overall, these are great machines. I have one running as vtools right now, and it's been solid for a couple years. Once vtools moves to an OS X/Intel machine, that QS will probably become my daily driver OS 9 toolbox.
