I've been running Aperture for two weeks, now, on a 1.67 GHz, 15" G4 Ti Powerbook with (only) 1 GByte RAM. I agree that there are times when more power would help, but not often enough that I work about it.
As an aside, I was swayed by the "bad press" Aperture had been receiving, so I initially held off on the purchase. During the first several hours after the install, I begain to wonder whether the nay-sayers were correct. However, once I got the hang of editing sequence (e.g., what modes to enter and what keys to use), I began to realize that this is indeed a slick product. I've always been a fan of both Adobe and Apple for their GUI ergonomics: this application excels nicely in that regard.
Back to the speed issue, I have not yet attempted to import to Aperture the 5000 photos I have in iPhoto -- at least not in one fell swoop. From what I gather, that would be pointless from a management perspective. I'm bringing in photo's one "film roll" (iPhoto terminology) at a time by importing the relevant folder from the iPhoto library.
I've brought in five rolls or "projects" (Aperture terminology) so far and printed books, via a PDF intermediary and a Windows box, to my Epson 2000P. The print quality seems to exceed what I was getting from iPhoto, perhaps because of the pixel resolution, though I may be mistaken in this regard. In any event, the prints looked quite good -- at least as good as anything I got out of Adobe Photoshop, Adobe Album and/or iPhoto.
Enough said. For those holding back on Aperture because they "only" have a Powerbook, I say: go for it.