Exactly the same here apart from I do my development work on it also (websites: photoshop cs2, macromedia studio 8, both plenty fast enough for web res production) and it's on Panther 10.3.9.
Same RAM, graphics card and a couple of hard drives.
Oh and mines a 17" Apple Studio Display.
I have my eye on a Mac Pro, won't be pulling the trigger until after WWDC though if at all. (May go top end iMac but already have a screen that's perfect for me and I like to buy for max longevity).
You can do a lot with a G4/400. A secondary machine (if you already have a better machine) placed strategically in your house for email web and music.
Server of course.
If you want to do some geeky upgrades, you have a lot of options. Processor upgrades go up to 2ghz, video card can be taken to a Radeon 9800 (so long as you have an AGP and not a PCI model). Hard drive controller for huge hard drive upgrades (they can't take over 120gb or something like that stock).