I bought a Cube sometime back with the intention of using it as a media player. I have a 17" Formac Gallery display to hook up as monitor for it. iTunes obviously isn't a problem, but I would like to playback decent quality video. It handles 480 HD quite well, but I would like it run 720, since the display res is good enough.
I figure I'm going to need a CPU upgrade to do it properly, but I don't want to spend hundreds on one. Any recommendations for the minimum required/cheapest options?

Gonna need sound too, should I hold out for some Apple original USB speakers, or just get an iMic or use Airport?