I've had quite a bit of response to yesterday's post about HD MythTV. Thanks for the input folks, you've pretty much answered all my doubts!
Cafuego told me he's having no trouble recording two HD streams over an NFS share on 100 meg ethernet, so local SATA shouldn't have any trouble. That's about what I'd expect too, though commercial flagging crunches the drive a lot. Spread between two drives with the new Storage Groups feature, I reckon it'll be fine.
Dave wrote to tell me he's happily watching HD on an AMD64 3000+ CPU and an Nvidia GPU. He points out that XvMC acceleration of MPEG decoding is important, which is the same as I've found with my silent SD front-end. One thing I've been wondering about is whether Australia's DVB-t solely uses MPEG or whether one of the other standards is also in use. The Wikipedia page mentions use of the H.264/MPEG-4 AVC codecs, though I suspect that's a very new thing and not used here.
Dave also points out that [wife] "Rozie never seems to get around to watching half the stuff she schedules", but that's pretty much the point of a PVR for me. Record everything that might, possibly be interested, but don't get too upset if it expires off unwatched. The occasional night you spend in wanting to blob out with lots of telly makes it worthwhile. The approach becomes "what's the telly got for me" rather than the old style "what's on telly".
Matt chimed in with some great info about the Panasonic LCDs I'm looking at. "I helped my parents pick out the 700A a couple months ago, and they (and I, when I'm around) have been really, really happy with it. The picture is extraordinary, and the signal processing (noise reduction, motion comp.) it does on the (crappy) foxtel and (sometimes crappy) free-to-air signals is amazing. Better than far, far more expensive 42 and 50 inch plasmas I've seen." Glad to hear it and that's been my impression from various forums and reviews around the traps. Not so sure about the motion compensation and noise reduction which seems to be the main difference between the 70A and 700A. Will have to check it out in a shop, side-by-side, I think.
Matt also suggest looking at Via Mini-ITX options which "are fanless and feature a DVI port". My SD front-end is an ME6000 and it's brilliant. Absolutely silent, which is ideal for the lounge room. The problem is, the newer EPIA boards that might have the grunt for HD look great, but I haven't been able to definitively confirm that the free drivers support the new chipsets, or that there's enough grunt for a 1080i stream. Anyone able to shed any light there?
TimC had some suggestions about fixing the green tinges on the old CRT. The green tinge appeared after we moved, and could well be due to proximity to speakers. He's suggested trying some degaussing or magnet actions. I might give it a go just out of curiosity, but the other half has agreed to an LCD so I'll still go down that path ;)
Craig described his VDR/Xbox running XBMC setup which sounds quite cool, but it wouldn't be up to HD and XBMC doesn't integrate with MythTV particularly. His comment about HD broadcasting in general rings pretty true though: "personally, i think that unless your TV is fantastic then you just aren't going to notice any difference between HD and SD except that the former takes up a *LOT* more disk space." I'm inclined to agree there, particularly if you haven't got a 1080i/p screen. 768 lines of resolution is certainly more than SD's 480, but it's not really that much.
I think I'll take his advice and see whether I care enough to upgrade from SD rather than dive straight into it. I suspect the improvement in quality won't be all that important, and the improvement just from having a bigger, better telly will be enough. For now, anyway.
So thanks for the responses folks. Most enlightening!