Another biased opinion...I believe, and always have, that the tower-based machines are overrated for TYPICAL home use. Yes, they have always been slightly faster than the iMacs due to L3 cache, and yes they are upgradable. And for some, that is a make or break decision.
But to me, a 17" wide-screen iMac with an FX 5200 Ultra and 80 GB (160 optional) hard drive and SuperDrive is just an amazing system right out of the box now. Neat, clean, space-saving. And equal in every way, unless you have a SPECIFIC need for L3 cache and slots, to the G4 towers.
For me, it only makes sense to buy a G4 tower over the new 17" iMac if you can answer YES to at least 2 of these 3 questions:
1 - Are you certain you'd rather have the ability to buy a G4 upgrade later rather than just saving up for the G5 now?
2 - Are you certain you want/need a screen larger than 17" wide-screen?
3 - Is it likely you'll actually need a video card more powerful than an FX 5200 Ultra over the next 3 years?
If you don't get at least two YES votes on the above, then I think you don't qualify as the type of person who really NEEDS a tower over an iMac. Most don't...they just want the "glory and power" of one, not being realistic about their NEEDS instead of their desires. And either way, if you talk yourself into a tower machine, then NOT buying a G5 now makes even less sense. Save for the G5, don't settle on a G4 tower if a tower is your requirement.
Good luck with your decision.