I guess that your desk is close to your tv so that you can wire the htpc to both.
My setup uses:
a Silverstone LC16B,
the ASUS P5KPL motherboard with integrated firewire, pci-e and a 1066MHz fsb,
a quad core cpu,
2GB of cheap 800MHz ram,
a WD Green Power HDD for lower power consumption and lower noise levels,
a fanless display card with 512MB for lower noise,
a dual HD tuner card and
a low noise psu.
XP MCE or Vista 32-bit will do.
I run VNC so that I can control it from any of three other computers.
I didn't realise there was a thread already on a similar topic.
I've just started a thread on whether I should build or by an HTPC:
I had an HTPC (still do) but I found this works much better for my purposes
QNAP Systems, Inc. ( NMP-1000 ) - Quality Network Appliance Provider
Sits under the TV, has all the right connections and connects to my PC via wifi. No more faffing around with codecs, screen resolutions, running wires from the PC to the TV etc etc... dedicated remote control too.. silent opertion, no need to apply security patches or worry about anything.
I wouldn't buy an HD-TV now, I'd get an NMP again every time.