Performance is indeed completely subjective. Actually, I take that back, it's truly objective, but we all interpret "fast" differently. Enough semantics.
I've run a bit of VPC6. On my iBook it is unquestionably slow as molasses. (640 RAM, G4 800). On my DUAL 1 GHZ 1 GIG RAM G4, WinXP was surprisingly "decent". I did fairly undemanding stuff like Access database and a few proprietary things that were similar. It was fine. Not something I'd want to do all the time, but decent. I've got to believe VPC7 on any G5 (iMac or otherwise) would be even better than my experience.
Interestingly, I found Win98 to run worse than WinXP. I guess I would have figured XP to be more bloated and slower, but not for me. Anyone else? Win98 performs much slower for me.
So, I'd run Word, Access, Explorer, ItsDeductible and similar apps without hesitation. Games, 3D or even Photoshop (not that I've tried) would not be something you'd want to bother with in VPC.