The G5s are awesome! Bonus getting an air cooled model, I've been reading a lot lately that the G5s (2.5 x 2, 2.7 x 2, and 2.5x2 x 2) that were liquid cool all seem to leak (like mine).
PCI-X is closer to PCI then PCIe. PCI cards are backward compatible with PCI-X, where PCIe isn't. It's completely different.
As for upgrades, go all the way to 8GB RAM, best single upgrade I ever made. Machine absolutely flew. You seem to have a pretty solid graphics card in there too, I had the 6800 Ultra, and it could handle everything I threw at it.
For wireless, I remember ordering bluetooth as a BTO option, so I'm not too sure if that's an upgrade you can just buy, but if a dongle works, that's great. Maybe grab a PCI USB card, so you have lots of ports, I think it's the NEC chipset you need to get, but I'd google that. I went threw 3 different cards before I found one that worked (
like this one). Airport was something I added myself, and was simple. Just bought an Airport Express Card. Make sure to have an antenna though, the G5 came with one, but since you got yours used, it may not have been included. It was a small "T" shaped gizmo that plugged in the back. You might be able to just get a standard antenna though, I'm not sure if the plug is a standard wireless plug, anyone know? The reception was pretty bad without it.
Throw a fast HD in there too if you can afford it, I had a WD black drive and it seemed pretty peppy. I don't know how a SSD would work in one, but pricewise it's probably not worth it.