This thread might give you a bit of insight, if you scroll down to the third section on HTPCs, but Ive quickly come up with this for you, case is personal choice and you could get a cheaper one, depends what you want, you already got dvdrw but if you want bluray playback youll need a bluray drive. Your cpu would be underpowered compared to everything else and this is what will effect the HD playback, so if youre struggling once up and running you may want to drop an e5300 in there
Antec Fusion Remote HTPC Case - Black (No PSU) £124.99 (£108.69)
LG CH08LS10 8x Blu-Ray Reader/16x DVD±RW - Black (Retail) £81.98 (£71.29)
Gigabyte GA-EG41MF-US2H Intel G41 (Socket 775) PCI-Express DDR2 Motherboard £53.99 (£46.95)
Samsung SpinPoint F3 500GB SATA-II 16MB Cache - OEM (HD502HJ) £41.99 (£36.51)
PC Power & Cooling Silencer 420W Power Supply £39.99 (£34.77)
OcUK 4GB (2x2GB) PC2-6400C5 800MHz DDR2 Dual Channel £39.99 (£34.77)
Sub Total : £332.98
Shipping cost assumes delivery to UK Mainland with:
DPD Next Day Parcel
(This can be changed during checkout) Shipping : £11.75
VAT is being charged at 15% VAT : £51.71
Total : £396.44
Edit: Also youll get more and better advice if you specify a budget