I have and still use a dual 2x2 G5. Worked with someone last night on their 2x2.5 G5. Your project demands will be the deciding factor. I'm looking to replace mine soon, but I'm still making money with it as we speak......
Tracks (as in audio tracks) will have the highest count if you can use a separate drive from the system drive to record and play back. A lot of DSP processing on the tracks can drag the CPU down. 88.2 and 96k (or higher) sample rates will drag things down. Try to stay with 44.1 or 48.
I've not had a project with up to 80 audio tracks - most of my projects are hybrid MIDI/Audio. 40 tracks for me is big project. That said, I've seen people have 10 tracks of something (say guitar) that I would put all on one or two tracks, automating EQ, compression (or whatever) that they created a new track for.
Anyway, in general, audio tends to put way less demands on the CPU than software instrument tracks. DSP processing can beat up a cpu - try things like putting space designer reverb on an aux and have multiple tracks share it.
I just finished a project on my 2x2GHz G5 (4GB RAM), and several of the songs had 60+ tracks. Like you, very few VSTi instances but many with more than a few built-in effects. No problems for me at all. I did find that freezing a few tracks here and there was useful but probably could have worked without that. I also have two UAD-1 cards in that box but didn't use them very much.
I've also been experimenting with using my MacBook Pro (Core 2 Duo @ 2GHz, 4GB RAM) as a node, and that works pretty well, too. I just bit the bullet and went with a refurb 8-core Mac Pro with 12GB of memory, so I'm anxious to get it up and running full bore because I expect a huge performance increase.
But I'll still keep my 2x2GHz G5 in my writing room. You should be fine with yours.