i beileve its a single it dont say it isnt
Even 480p can sometimes be a little choppy on my dual G4 when streaming with Flash. I suspect that 240p and 320p will be fine and 480p will be somewhere between watchable to fine in Flash. Of course, the better YouTube experience would be with MacTubes set to QuickTime.
A 9600XT is plenty for 2D video. It's not the video card that'll be holding you back.
Unless you're getting a great deal, I'd hold out for a dual CPU machine.