The 17" iMac is a dream to use... and if you get the wireless keyboard and mouse package (for only $100, $90 edu) it will be extremely portable (just not useable on the go). The G5 SCREAMS (with sufficient ram):
oh, and the 15" PB will have 54 MORE horizontal (the more important kind) rows of pixels then the screen you have now... So if you do go the 15" route, you shouldn't be disappointed. The lighted keyboard must be a nice perk as well.