csc2000e

What kind of budget are you looking for?
I would recommend a 9400gt: http://www.newegg.com/Product/Produc...82E16814121292
They should decode blu-ray quite nicely. I had an 8200 integrated motherboard(which is not nearly as good) and it ran HD content perfect. So this should work great. I don't know how well ATI cards perform for HD video but I'm sure they work great too.
As for the processor, it really depends. If you are pretty much going to be watching HD content on this computer, then much of the load of that stuff will be put on the graphics card. So your processor doesn't really need to be uber good or anything.
http://www.newegg.com/Product/Produc...Tpk=19-103-678
Something like this should work fine. If you have a larger budget, then I can recommend some other stuff, but this is what I would do if I was building an HTPC on a budget.