go for iMac/MBA
I recently upgraded my kit from a G5 MacPro and a 12" Powerbook to a 3GHz iMac and a 1.8 Ghz/64 GB MBA. I couldn't be happier with them. I had worries about the reflections on the iMac, and the truth is it's a complete non issue. As for the Air, reading these forums I was really wondering whether I'd made a horrible mistake, but again, my machine is fast, silent, and yesterday I callibrated the battery and to run it down played videos on YouTube for half an hour and it was absolutely fine!
As for the amount of memory...if you reinstall the OS and leave out things like Printer Drivers for printers you won't meet, and odd languages and GarageBand etc, and then you run Xslimmer to take out the PPC code, you end up with plenty of memory for a machine out on the road. I have Word, Excel and Powerpoint 2008 on mine, with Pages and Numbers, Aperture, Xslimmer and Starry Night, and I have 44GB free. That's plenty for a decent selection of music and photos and enough documents to keep anyone busy.
So what you do end up with is a light machine that doesn't weigh your bag down, is powerful enough for most things you'd want to do away from home, and a beast of a desktop for any hardcore video work etc.
Good luck!!