Sorry to hear that you have had speed issues with Lightroom. You are the first I have ever heard that has had this problem - most complain that Aperture is way slower than Lightroom.
I offer the following advice:
1 - check to see that you have at least 50% of your hard drive space on your G5 available. If you are working with less than 25% unused that can slow down all applications especially these.
2 - make sure that you Render the 1:1 previews after importing images into Lightroom. It doesn't happen auotmatically and this will greatly speed up everything in lightroom.
3 - I've used Lightroom with a Nikon D2x (12 MP) and a Canon 1Ds (16 MP ) so unless you are using a MF 39 MP back this shouldn't matter too much.
4 - if you have your lightroom Libray/cache on an external hard drive (USB or Firewire) this can massively (i.e. BIG TIME) slow down everything. Put the cache on a faster drive with a SATA connection if need be or better yet leave it on the G5's internal drive.
I have the exact same computer as you do and have experienced very snappy and fast movement in Lightroom - faster than any imaging software I've ever used. So I don't know what to tell you otherwise.
In terms of the user interface, well that is personal preference - go with what works for you. Happy hunting...
Cheers, Michael Clark