We can argue until the cows come home without getting anywhere about whether top of the line PC's or Apple's are faster, though I think PC currently have a distinct advantage, but there is no question at all an 800MHz G4 w/ 256MB of RAM and a GF2 MX will beat a PIII 800 128MB with shared video running practically anything. Plus the Imac is upgradeable to a 1GB of RAM.
As to which one I would rather own, if they both ran PC software (Imac never will, but hypothetically), I would much rather have the IMac. It's faster, more upgradeable, has a CDRW or even a DVD-R drive, a larger hard drive, plus firewire ports. The Imac beats it in almost every category. At no point have I ever sat around and wished my PC had an FM only radio in it. Only problem, the top end Imac probably costs more, and the biggest problem, it's an Apple, which means it doesn't run any software.