Even with the OS you probably won't be even close to 800 unless you buy an absolutely extravagant case.
Rough setup
Athlon II X2 CPU $60
880G Chipset Mobo (Or 785G if you find a good deal) $90
2GB DDR3 (DDR2 if you go 785G) $100
1TB WD Caviar Green $70
Corsair 400CX PSU $30 (AR)
Win 7 Home $100
So about $450 + case/upgrades, but thats a bit of an overestimate and isnt including any combos or other rebates. It'll likely pan out to a bit less.
Possible upgrades; a tri or quad core CPU if you're encoding your movies as well, a blu-ray drive, a modular PSU, a 2TB hard drive, a discrete GPU or 4GB RAM (I went with both, use my box for emulation and moderate gaming). Depending on your case you also might need to consider a non-stock heatsink because many HTPC style cases will restrict the amount of room you have to work with above the CPU.
Off hand I'd say the Silverstone GD05B is a great desktop style case, really like mine. But a little on the expensive side.