Speaking ...
... of capturing - storing - the raw stream of h.264/mpeg2 hd movie data on your harddrive and editing it with any HD capable movie application, I would say yes.
But playback, no way, if it is 1080i/p material. For that you need a G5 with at least 2GHz just to be on the save side. 720p is possible without framedrops on any G4 faster than 1Ghz, I think, with lots of RAM included.
Encoding DV into h.264 is taking ages right now with pure processor speed, even in PC land.
E.g., I myself can capture mpeg2 transport streams from my cable tv via ethernet to my iBook G3 500 Mhz without any problems. I can edit it, store it, transfer it, but playback is always choppy as hell on that machine. Only if I transfer it to DVD, I get perfect playback, but only because Apples DVD player app does use hardware accelerated mpeg decoding from the graphic card.