I would recommend more RAM, which you are already doing, and one or more faster hard drives. See
this thread. I achieved a significant speed up of my Mac Pro by installing a faster hard drive than the one it shipped with.
After some research on the web, it is clear that going to a two disk RAID-0 configuration will take this speed increase to the next level. I don't know if the 2.7 GHz G5 supported RAID or not, but if it did, this would be an interesting option. It is expensive though - Apple's current Mac Pro RAID card is over $900. Apple hardware remains "dear"!
This thread shows you how you can do this via external drives as well. The results do look VERY impressive. A faster disk subsystem definitely speeds up your overall machine experience.
Do bear in mind however that performance is a multi faceted beast. Once you optimize the disk element of this, your top performance will become bounded by the ability of your CPUs. Unfortunately, that will be as fast as you can go. You are running the fastest G5s out there.