I've been playing around with YT this evening on one of my Cubes. This is my bare bones basic, 450mhz model. It does have 1.5gb of RAM and a Radeon 7500 driving a 22" ADC Cinema. I haven't(yet) installed Leopard, but it runs Tiger fairly well.
In any case, it's connected directly to my router via wired ethernet. In TFF, Youtube is actually useable-if not totally pleasant. I've found that the key is to let the video load for a minute or two before trying to play. Otherwise, it stutters badly(on the video, not usually the audio). Once loaded-i.e. if replaying a video, it's about 95% okay.