You could just follow the M$ mimum spec for the media center 2005
Optimal spec to play hidef tv at 1080i with surround sound
# Windows XP
# Windows Media Player 9 Series
# DirectX 9.0
# 3.0 GHz processor or equivalent (Any AMD or P4 false economy to get either the sempron or celeron, although tbh the sempron could handle media stuff quite easily.)
# 512 MB of RAM is enough (my last pc had 512meg and handle all media stuff etc fine, you can always upgrade to another 512MB at a later date.)
# 128 MB video card (Nvidia 6600 should do the and job less than $100 or £100 or 100 something euro, but as its a media oc i would look around at reviews for the one with the quietest fan.) My 6800GT is a noisy bitch and is soon to have its cooling system ripped off and replaced
)
# DVD drive (Nec 3520, best dvd writers)
# 1920 x 1440 screen resolution (that refers to monitor or TV)
# 24-bit 96 kHz multichannel sound card ( Most motherboards have on board sound and
# 5.1 surround sound speaker system (There fine for media stuff, although MSI do have creative 7.1 chips built in which is quite nice)
Avoid a cheap ass psu, by something by entec, enermax, akasa, seasonic , hiper, there all pretty quiet too. Some of tehm are fanless so 100% silent
Hard drive wise seagate (5 year warrantly very reliable ) pretty quiet or Samsung for supreme quietness.
Case ig you can get one with 92mm or 120mm fan slots means you'll be able to cool your setup nice and quiet..
Have a look at the antec range they do some nice quiet case designs and come with a decent psu.
All depends how much you want to play. Just specced up a collegues PC at work to a pretty much a similar spec and came to £508 with a AMD 3200 64bit cpu and 1 gig of ram.
Could probably knck another £50 if i shopped around as i was just using one shop. But all parts were quality bits.
drop yourself down o a sempron and 512 meg and you be looking at the region of £400