The latest versions are apparently fine, and terrifically priced too, bootable in OS 9 (which I like) and minus Bluetooth, Firewire 800 and something else I don't remember and don't need. But I would get the least about of RAM and buy your own. You'll save about hundreds right there.
Only the dual G5 is worth considering. It's about twice the speed, but twice the price too. Plus *not* bootable in OS 9, no room for a second optical, and no room for more than 2 drives -- factors that are a big minus for me.
What I'm a bit confused about is why people are complaining about the lack of internal space for extra drives. There's this neat technology called FireWire that allows you to hook extra external drives up to your Mac. Plus there's the Xserve RAID if your storage needs are truly massive.