I've just finished building my PC and am very very pleased. I picked up a cheap refurb Dell 5150 from ebay and chucked in another 3 hard disks (4 in total with a bit of hacking and power splitting) and a dual DVB-T (digital TV) tuner PCI-E card. All running Windows Media Center 2005.
It rocks!
I love this new BTX motherboard format. The PC (even with the 4 drives) is almost inaudible!
Here's the spec for all you geeks:
Pentium 4 HT 630 3.0Ghz (2MB Cache)
160GB SATA drive
plus 2x250GB and 300GB Maxtor hard disks
(960GB in total!)
Terratec Cinergy 2400i dual DVB-T digital TV tuner PCI-Express card
ATI X600 256MB
NEC DVD-RW
IDE RAID card
Soundblaster Audigy 2 Platinum Pro with breakout (sounds great with my guitar)
Windows Media Center 2005 OS with IR sensor and remote