Not really. 1.6GHz Core1 chips and 1.6GHz Core2 chips (the original mac mini and the original macbook air) outgun the G5s in memory and processor performance. Like, all G5s except for the dual 2.7 and dual-dual 2.5. It's only when you can install a discrete graphics chip that's got like a gig of memory, and/or when you can install like sixteen gigs of ram that it's actually a better idea to get a G5, especially now that snow leopard and any/all 64-bit apps (Adobe's apps specifically) will be Intel/SL only.

AdobeCS5 will be intel-only, as an example.

The mini will act faster if you were able to put a 3.5-inch 7200rpm or 10,000rpm hard disks and a bunch of ram into it, but at 2.0GHz it definitely outguns any/all G5s in terms of CPU calculations, and with 1067MHz or 1333MHz ram, it definitely outguns a G5 in raw ram performance.