Logic 7 upgrade to 8, on G4 PB and/or G5

Greetings! I have for years been using Logic 7 on a G4 Powerbook, (1.5GHz, 1.25 GB RAM, 10.4.11) and have just acquired a G5 desktop, and am about to get a Logic 8 upgrade (Version MA800Z/A) from eBay.

I was assuming I would only want to install the 8 upgrade over an install of 7 on the G5, record on the PB and Save As Project for mixing etc on the G5, but I have read reviews suggesting the CPU load in 8 is not that much greater than 7.

So would it make sense to install the 8 on the PB as well, bearing in mind I wouldn't be asking much heavy lifting from it, just basically the original acoustic recordings? Or have folks found 8 to tax older machines too much?

Many thanks