Was in almost exactly the same situation myself a few weeks ago - PC for work, 12"PB for portable and decided to make the switch. Went with the 1.25 17" iMac and am absolutely delighted.
Really depends on what you want it for, I am doing moderately intensive Word + Excel + graphics stuff (not pro graphics though) and really speed is not an issue - and I have been a PC user for years previously.
Am no expert regarding Macs, but my thinking was to go with the iMac for home and then make the move at work in a year or so, going with a dual processor G5 - I reckon that the big advantages and benefits will be seen there. A 1.6 G5 will be nice, and faster, but the real leap (and maybe bang for buck) is going to be in the duals.
But for general home stuff, well, I really am glad I switched.
The lack of physical upgradability of the iMac is undeniable, but then again, discourages amateurs like me taking it apart for a bit of a tinker!
Anyway, sorry to go on, but I had found this forum very helpful before buying and understand your dilemma!
Suspect you will be happy with either at the end of the day...