To my surprise, Aperture runs on my Cube, and runs tolerably. I had to hack it to get it to run, given that the Cube does not have official Aperture support, but everything works just fine (albeit fairly slowly). This Cube does have a 1.7 GHz G4 which is adequate as far as G4 speeds go, but the real surprise is that Aperture has no problem with this Cube's GeForce 2 MX, a GPU that is several generations behind even the GeForce FX 5200 (which is also unsupported). The GeForce 2 MX does not even fulfill the hardware requirements for Core Image's GPU-based acceleration. That likely means that Aperture supports Core Image's Altivec fallback on the CPU, despite the fact the application is built never to run on a machine like this. At least this GPU has Quartz Extreme support though. One wonders if Aperture could run on even an older GPU like the ATI Rage 128, which does not even support Quartz Extreme.
I won't detail the Aperture hack here, but suffice it to say that there are explanations available elsewhere out there online, and a valid serial number is still required of course. For the sake of all the people out there with adequately spec'd G4 Power Macs, I just hope that Apple doesn't choose to disable the hack in the 1.0.1 update to Aperture.
Now I said it's tolerable on the Cube, but in truth maybe I think it's tolerable just because I didn't test it for too long. Fortunately, I already have a faster and fully supported iMac (G5 2.0 & Radeon 9600 128 MB). Aperture is not particularly fast on my iMac either, but at least it's a noticeable improvement, although nowhere near as fast as the dual G5 2.3 GHz Power Mac with GeForce 6600 256 MB I had the chance to test out with this application.