I have this exact spec - except i have a much smaller case.
Antec Fusion HTPC Case - 430HE Watt PSU Antec Fusion HTPC Case - 430HE Watt PSU (£115.14)
Intel Core 2 Duo E2160 "LGA775 Conroe" 1.80GHz (800FSB) - Retail
(£48.16)
Abit Fatal1ty F-I90HD ATI Micro ATX (Socket 775) PCI-Express DDR2 Motherboard
(£68.14)
OCZ 2GB (2x1GB) PC2-6400C4 Dual Channel Platinum Revision 2 XTC Series DDR2 (£27.01)
Total: £258.45 including VAT. That leaves £240 for storage, remote control TV tuners, and anything else you might want.
The motherboad has true onboard HDMI and optical SPDIF outr, unlike the ASUS AM2 one which uses one of the PCI-E slots. If you want to go AM2, Gigabyte do the best one (
link). It too has onboard HDMI and optical, plus it has a better Realtek sound chip. The best they do at the moment.
PS:
Mine is playing the King Kong HD-DVD right now at 1080p/24 on my 40" TV. What is interesting is, I was running 1GB until today and my CPU load has dropped from almost 100% @ 2.4GHz, down to around 50%-60% at only 1.8GHz since I replaced it with 2GB. Best £25 I've spent in quite a while.