Definately check out the link from Djlackey5. The AVS HTPC forum has wasted many of my hours....With the proper searches you'll find everything you need to know.
A quick recommendation or two: For the audio side I am quite fond of the MAudio brand of soundcards. High Quality DD and DTS passthrough, and believe it or not the analog outputs aren't bad (considering the RF nightmare that is a computer). Radeon based video cards produce amazing results (avoid the 8500 [i think that's the model number] -- it has a gamma problem). With spare parts and few hundred bucks (the above mentioned components) I built a HTPC that blows away my progressive scan DVD player (panny RP56). This is, of course, not including the man hours that it took to configure every thing correctly (and I used to build and fix PCs for a living).YMMV.
Hope this was helpful...