Thought I better chuck this on a new thread - seems people are having mixed results decoding the 720p signals with ATI cards - be interested if anyone has had luck with AGP based cards - as I'm about to sell up and start again.
My HTPC setup is AMD Athlon 3400+ 1Gb DDR400, XPC SK21G box, Vista home prem, Cyberlink DVD 7 Ultra, GBPVR, hauppauge HVR1100. It chokes on the test signals currently found on  DVB-T TV1, TV2 & TV3 - 100% CPU utilization & choppy playback through a panasonic 37" plasma @1020X700 (adjusted to fix overscan).

I am using the latest catalyst drivers and avivo downloads, the VPU does not seem to be pulling its weight in decoding despite all specs saying that it should be, leaving the CPU to try and grunt it out. If there is a solution out there, I certainly haven't been able to find it. I have checked "hardware decode via avivo" in the cyberlink options - no difference in performance whether this is checked or not.

I'm running through the process of selling existing rig and putting together a more serious contender but if it is money I don't need to spend, that works too. 

The CPU is the fastest 754 pin CPU that the box will take, so that isn't an option. In all other respects, the system runs well & is a reasonably speedy gaming rig.