Thanks Redwight for all the information.
Looks like the 3450 might be the way to go then as its going into a HTPC with no likeihood of gaming happening on it, and if it dosn't need extra power than thats also an additional bonus - even with using a 420W PSU.
If I want to game, I'll just use my laptop - its got Go7600 with 256MB dedicated RAM, so its more than adequate for the gaming I do, and it looks just as good on the 42" LCD TV
