You've probably already discovered it, but just in case, as a heads up, this is not, by any means, a modern computer. At absolute minimum this machine is sixteen years old, and you can not think about or treat it as if it were a modern computer or as if there is any hope at all that you'll be able to do modern things on it. This largely includes the Internet. Where things work (tenfourfox) they often work poorly because of how slow these things are.
In terms of upgrades: There's not an awful lot. This machine is relatively inflexible compared to what came both before and after it. You can basically only do:
- Storage (which you have already done)
- RAM
- Graphics (although there's a much narrower band of compatible/suitable cards)
- Whatever else fits in the PCI slots and has drivers and might fill a need/desire you have.
It can't hurt to put a bit more RAM in it, although it's
probably not worth actually maxing the RAM, 64-bit graphical software didn't really come to the Mac until much, much later and so to max out, especially the later/faster G5s is almost entirely performative or because you want to keep several disparate task sets in memory.
Graphics, I'd say only to bother with if yours is failing or you need something like dual-link DVI to run a 27-inch 1440p display. (or you otherwise want different outputs, I suppose.) Every G5 graphics card is Core-Image capable and so they'll all run 10.4/10.5 "fine".