Vista MCE, 690G and live HDTV?

I just built a new media center with Vista Ultimate based around an ECS AMD690GM-M2 motherboard and an Athlon X2 3600+ Brisbane. Basically I'm trying to shoot for the lowest end possible components that will still be capable of doing hi-def, and I thought this would be it. However, my actual results have been that while playing back recorded HDTV is fine, and while watching live SDTV is OK, live HDTV stutters. Have I underestimated my needs on the GPU/CPU front? My Vista scores for the two video items are 3.5 & 3.1 respectively, with all other scores around 5+.

Other potentially relevant info:
single SATA 300 hard disk (non OS drive is ordered but not yet installed)
2GB PC 6400 RAM
Avermedia m780 hybrid PCIX tuner
Nvidia Purevideo decoder
Vista Ultimate
Catalyst 7.6

Thanks in advance for any tips or suggestions!