Are you going to network it with other PC's in the house? If so you may want to consider just having a small Hard Drive (say 80GB) in the htpc and using the other computer to stream media from.
Also if you already have a spare copy of xp have a look
here for some media center software. Its free and vey good.
How much are you thinking of spending and what sort of case do you want?
Something like this may be the sort of thing you could look for:-
HD-023-SA Samsung SpinPoint P HD080HJ 80GB SATA-II 8MB Cache - OEM (HD-023-SA) 1
£29.95 £29.95
CP-117-AM AMD Athlon 64 3000+ Venice 90nm (Socket 939) - Retail (ADA3000BPBOX) (CP-117-AM) 1
£61.95 £61.95
MB-119-AS Asus A8N-VM CSM Micro ATX (Socket 939) PCI Express Motherboard (MB-119-AS) 1
£46.95 £46.95
MY-005-GL GeIL 1GB (2x512MB) PC3200 Value Dual Channel Kit CAS2.5 (GE1GB3200BHDC) (MY-005-GL) 1
£54.95 £54.95
CD-043-LG LG GSA-H10ABAL 16x16 DVD±RW Dual Layer ReWriter (Black) - OEM (CD-043-LG) 1
£21.95 £21.95
HS-010-ZA Zalman CNPS7700-CU Ultra-Quiet CPU Cooler - Retail (HS-010-ZA) 1
£19.95 £19.95
CA-011-SV Silverstone Lascala SST-LC11M HTPC Case (Black) (CA-011-SV) 1
£139.95 £139.95
GX-074-LT Leadtek Winfast DTV1000-T Digital TV Tuner Card (GX-074-LT) 1
£32.95 £32.95
Subtotal £408.60
VAT £71.51
Total £480.11
The HD may be too small if you dont want to network, if it is look to get the 250GB samsung instead. The case is a slimline one, quite small and expensive but it includes an integrated Ir reciever, software, remote and 300W PSU