I think it is way overkill for a HTPC. A P4 can do a good HTPC no problem.
Also your video card is overkill and wont do you any good. Video cards like that are meant for gaming and a high number of pixel shaders and such wont help you rendering video.
If you are wanting to record 2 HDTV streams at the same time, while outputting an HDTV stream, you may need some good CPU power more than a P4. And today, the best bang for your buck desktop wise is a cheap Quad core.
I have an HTPC running a 2.2Ghz P4, with 512Mb RAM, and outputs basic HD (component/composite I get them mixed up. It is the 5 pronged ones with 3 video 2 audio) and it never skips a beat. Total cost including case/psu was under $200. Internal storage of 20Gb but streams media from my server PC (2.5Tb of space).
So a media PC doesnt have to be so complicated.