RAM is about it. I forget exactly how much that model can take, but I would assume it's at least 6-8GB.
Nothing else is going to really be worth the effort. Outside of that, if any of the newer versions of the software you use improve AltiVec support on the G5, that would probably help, but it's unlikely and probably very expensive.
You could try upgrading the CPUs, and that system MAY accept dual core G5 units, but the bang for your buck ratio would be very very poor I suspect.
Ultimately you have to accept the fact that the system is going on, what, like 5 years old at this point? The world has moved on, and this system is no longer the top of the line power house it once was. Even a $600 Mac Mini could probably give it a good run for its money. More RAM will help to a degree, and it's relatively inexpensive, but don't expect miracles.
I have a dual processor G5 Powermac with the following factory configuration that I use for Photoshop and Lightroom work. Aside from replacing it, what hardware could I upgrade to improve performance?
CPU Speed: 2.7 GHz
Memory: 4.5 GB
Hard Drive: Serial ATA WDC WE2500JD 7200 RPM, 232 GB storage
Graphic card: ATI Radeon 9650 with 256 VRAM
Monitors are an HP w.2408 and a your standard HD second monitor