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.
Kent