I have built a couple of HTPC's in three weeks, the first attempt was a 2.8 P4 no HyperThreading on a Shuttle XPC case, with PowerColor 9550 and 512MB RAM. Running zoom player, ffdshow with avisynth filters produced a lot of stuttering so I have to reduce the settings. So this config is not quite good.
After two weeks, I bought an MSI P4N SLI-FI with a 3.2GHz P4 with HT, 1GB dual channel DDR2 RAM, Nvidea 6600 GT (for now only single card, but soon would be on SLI), 550 watt Enermax PSU on a spare Lian Li case I have lying around. Performance is great but the NOISE is very annoying (P4's and the Nvidea 6600GT generates so much heat I need to use a total of 6 fans on the case including the PSU fan!)
I have a 64bit AMD Athlon at work and it runs very, very cold compared to my P4.
If have to build another PC for HTPC, I would surely go for an AMD X2 (which would be my project soon!) Of course, an X2 is prohibitively expensive. I think the Athlon 64 3200+ should do the job. If you will be using zoom player, ffdshow and avisynth, just keep your filters, denoising, sharpening and resizing moderate.