Both my dad and i have HD sets but no blueray players for HD content and since i download a lot of HD stuff, i built a media PC. I wanted to have a fairly good one so my spec was...
core 2 duo 2.13GHz clocked up to 3GHz, which raised the FSB to something like 1500MHz.
1Gig DDR2 @ 800 MHz
ASROCK 4CORE FULL HD motherboard - included ATI graphics chipset that was rubbish - corners of movies tearing etc! so had to buy a
fanless ATI HD2600 XT PCI-E with big heatsink <- possibly slightly overkill for a media pc though.
mine plays virtually anything using the excessive (and inefficient) KLM codec pack.
My dad does a lot of work with PC's and as such has quite a few older spec'd ones around. After reading around the net i thought that we could build one for him using some of his older components. We settled on ...
P4 2.8GHz, 800MHz FSB - all stock frequencies
ASUS P4P800 motherboard
1gig DDR1 400
ATI HD2600 Pro (AGP 8x version)
which has been a bit of a headache to get it to play HD stuff properly (even 720p!) With the best codec properly configured for the type of movie it would play ok (including some 1080p stuff) but it was no where near as simple as just installing one codec pack and not having to worry about it! - not best practise but quicker when you don't know what you're doing!
Once i've figured out a fool proof way to determine the codec type required, the best one available and how to configure it for each HD movie i have then i'm sure my dad's media PC should play most stuff
I also wanted to run a myth tv box but had so many problems installing the ATI graphics drivers in linux that i gave up. Nvidia cards have much more support in linux i feel.
If you're buying from new -personally- i'd go for a core 2 as you'll futureproof yourself and on ebay my core 2 was about 50 quid compared with a P4 for about 30.