A dual 450 should work -- Miglia recommends at least 500Mhz, but since you have two it's kinda like 450 + 450 = 900Mhz . You don't use much of the mac's processors when you watch or record TV, but watching already recorded shows or watching TV with an offset (aka, pause for 10 seconds, then hit play again) is what will suffer on a slower mac.
You will have to get a PCI USB 2.0 card for the powermac g4, as none of them had 2.0 on-board. Iogear makes a solid card for ~$20.