I can now answer my own question
The 20" with 2GB of Ram (and 256MB graphics) was VERY quick compared to my 1.42Ghz mac mini with 1GB ram.
Office under Rosetta doesn't give any clues that's it's running under Rosetta at all (again compared to my mini of course).
MacMame works a treat under Rosetta for old games. Auto frame skip kicks in like a good 'un for newer stuff - so playable, but looking forward to intel libraries for compilation.
DreamWeaver seemed a little sluggish, but pefectly useable.
Umm, couldn't find any universal games that i wanted to play to try. The supplied board games ran fine, but then, they ought to! The demo of X2 - The Threat was unplayably stuttery. Verdict is probably that even with 256mb graphics games are probably a no go under rosetta but i'd definately have to try more to really pass jusdgement on that.
A word of warning. The screen's are a horrible grainy mess. Mine's on it's way back with an RMA number due to pixel problems, so i kind of reserve judgement on that too, but i have my doubts that the two would be even remotely related.
The extra screen real estate oof the 20" was fantastic. Had a bunch of windows open while compiling mac mame and still had more room!
All in all, the screen is a dissapointment, but i really can't wait to get my replacement and start again
PS: Oneagle: I got your message about the PSP trick, and i hear ya. Normally i'd be up for having a go but for ¬£1500 Apple can sort it, not me. I think it's wholely reasonable to have one that's 100% from the start - LOL!