I would go for a Sawtooth for your purposes. I have 2 of them (got them both for $10 each at a recycle center). One of them I have upgraded with a sata card, USB 2.0 card, a gig-e card from a G4 xserve, a Geforce 6200 and maxed out to 2GB of ram. There's 4 HDD's crammed in there, 2 of them are sata. I stuck a dual 450Mhz CPU card in it. It runs Leopard mainly, and it does it very well. I'll browse the web on it sometimes, even if it isn't the fastest, its definitely doable.
A Mac Mini G4 won't run Mac OS X Server 1.2, and they also have only 10/100 ethernet which makes them poor servers depending on what you plan on doing (at least with my sawtooth I was able to install a gigabit card). They can also be had for very cheap. Excluding everyone on eBay who seems to think their old mac is worth a small fortune... Shop around at local recycle shops/e-waste centers, and craigslist.
Sawtooth's have an advantage over the other G4 towers up until the MDD. 4 Ram slots. You get 2GB instead of 1.5GB, which makes a bigger difference now than it did 10 years ago. The only disadvantage to the sawtooth is the 2x AGP slot, and the 100mhz system bus. There are also some pretty fast CPU upgrades if you're lucky enough to find one. If you use a flashed PC card you can still fit a radeon 9800 pro in there if you wanted. And the bus speed hasn't bothered me that much. I did a geekbench and xbench on my sawtooth, and my QS 2002 single 800mhz, they were almost equal, the sawtooth was actually faster irc. I'd post the results but I'm at work and I don't have them.