Hmmm.... a home cinema....low power...quiet...the last PC case you will ever need.
A small HDD until the 1TB ones come down in price.
You don't need i5, i3 may be, you don't need SSD you're not doing anything fast here.
You don't need more that 300W here either. The 450W PSU would allow a gaming gfx card to be added in the future.
The discrete card here will ace bluray playback. The motherboard does all the hd audio you will need.
This also includes 2 AMD baseball caps to wear whilst the team builds the rig and you load the OS
Fractal Design Define R3 Midi Tower Case - Black Pearl £84.98
Microsoft Windows 7 Home Premium SP1 64-Bit - OEM (GFC-02050) £79.99
Samsung SpinPoint F4 320GB SATA-II 16MB Cache - OEM (HD322GJ) £69.98
Samsung SH-B123L/BSBP 12x BluRay ROM / 16x DVD±RW Lightscribe SATA-II Optical Drive - Black (OEM) £54.98
AMD Athlon II X2 Dual Core 250 3.00GHz (Socket AM3) - Retail £53.99
Corsair Vengeance Arctic White 8GB (2x4GB) DDR3 PC3-12800C9 1600MHz Dual Channel Kit £45.59
Gigabyte GA-M68MT-S2 (AM3+) Motherboard - Retail £33.98
Lepa W-Series 450W '80 Plus' Power Supply - with FREE LEPA Hard Drive Enclosure £43.99
BeQuiet Dark Rock Advanced CPU Cooler (Socket LGA 775 / 1155 / 1156 / 1366, AM2 / AM2+ / AM3 / 754 / 939 / 940) £41.99
MSI GeForce N210 1024MB GDDR3 "Low Profile" PCI-Express Graphics Card £24.98
AMD Green Baseball Cap x2 £3.98
Total : £559.87
Cast an eye over the memory compatibility and make sure the PSU choice is quiet.
This leaves loads of cash for a wireless kb, mouse, and copies of Ghost, Dirty Dancing and Megamind for your viewing pleasure.
Alternatively use an i3 2100 and an Intel MB and burn a few quid more. I do not know how good the onboard graphics handles Hi Def but someone here will.