The iMac isn't designed to be user-upgradable - other than the RAM slots underneath.
To upgrade the internal hard drive you'd have to take the whole machine apart, which would invalidate your guarantee/Apple Care agreement, and be a nightmare to put back together unless you're a very experienced hardware engineer/modder.
Cheaper to get yourself an external USB2/FW400/FW800 external drive, or build your own DIY solution. Then you've got a spare drive ready for OSX 10.5's TimeMachine backup too.
Well, Apple themselves supply instructions on how to do this on a 17" G5:
http://manuals.info.apple.com/en/imacG5_17inch_harddrive.pdf
Google search: "apple imac 17" replace hard disk"
Seems to imply that it won't invalidate the warranty either... I was planning to upgrade my measly 250GB to 500GB once the prices of SATA2 500GB's have dropped.
I'm sorry, but this is not true. I easily replaced the drive in my 17" iMac G5, and the new ones are not significantly different. It's no trouble at all if you know how to use a screwdriver.