I'm running:
3000+ Venice @ stock
2 x 512mb ram.
MSI K8NGM2-FID board (6150/430 chipset, with HD audio etc)
Foam mounted Samsung Spinpoint 160gb (Soon to become 400gb - now film four is on freeview 160gb will no way be enough hehe)
Terratec 2400i Dual tuner Freeview card (I was using 2 x Compro T200 which although were good had a few niggling issues that i don't get with the terratec)
This is cooled by just 2 x 5volted Nexus 120mm fans (one exhaust, one PSU, CPU passive). They spin at less than 500rpm :O It is inaudible even in the deep of night.
After trying a LOT of different hardware in the quest for the perfect media center I'd say the above is just about the ideal and most cost effective system. It is super low power - less than 65W idle and less than 100 full load. the S3 suspend function works flawlessly on this motherboard too - waking up to record scheduled programs etc. It uses just 7W in the S3 state. Stability is perfect too with the right drivers, no crashes, no BSODS etc and the machine is lucky to get a reboot fortnightly. (Be wary of the latest nv 91.xx gfx drivers, the nv ide driver and don't install nvfirewall)
A few things I have found: There is no need to use a discrete graphics card - I saw no real improvement with a 7600gt in there, just more heat and power, however you do definitely want to use 2 sticks of ram in dual channel mode if you are using the on board gfx.
A dual core chip isn't really needed, although at the current prices I'd be tempted if you plan to do anything with the machine at the same time as recording tv etc.
I now use the powerdvd 7 mpeg2 decoder rather than the nvidia one. The only way to stop the stutter issues with it is to turn off smart interlacing - which seriously degrades picture quality in some situations.
Incidentally - you can certainly build a usable media center machine with a lower spec - I initially started off with a 2500+ barton @ stock and 512 ram, but with the current prices of A64 chips and their low power (therefore quiet) characteristics they are worthy choice. I'm sure some fanboys will scream conroe, but there just isn't the need to spend that much money to build a decent htpc.
MS Media Center 2005 is typical MS software, it can have you pulling your hair out when trying to get it set up, but once you find that sweet spot it works flawlessly.
Marc
Thanks for the info guys.