I've been running my own PVR for years now, recording standard definition DVB-T in the UK on a box based on WinXP and GBPVR. Basically, I need advice as to what CPU I should be upgrading to to do what I do now but with high def DVB streams. What CPU should I be looking at to put in my box that will allow me to record two high def DVB streams and watch a third, all at the same time? I currently use an underclocked Athlon XP-M@1.6GHz to do this with standard def streams.
I will of course also be getting a new mobo, RAM, and probably DVB cards if it turns out my Hauppauge PVR-250s won't handle it.

Really any commonly available dual core CPU should be plenty fast to do what you want as long as the tuner cards you install have on board hardware MPEG encoders.

I recommend one of the Hauppauge (http://www.hauppauge.co.uk/) cards.