For just a pure HTPC setup the new onboard video is ideal

The new ATi 780 motherboards do provide full HD decoding but I have not seen if it supports full picture processing in the same way the HD3470 and above PCIe cards can. Also the inbuilt GPU is overclockable to the same speed as the HD3470 so should have the performance to do it, it just needs extra cooling / fan assistance for the overclock.

The new 8xxx nVidia motherboard is reported to also have full HD decoding, this has not been released and I don't know of any benchmarks for pre-release ones yet.

Intels new integrated chipset (the one after the G35) is also reported to support full HD decoding. Personally I will only believe it when its released and reviewed.

Alot of the problem comes from the chipset manufacturers, alot of them stated that their integrated chipsets were able to play back HD content but all they could really do is play back video at the needed 1920x1080 res with no hardware assistance. The newer chipsets are now providing this support.