A DDR3, Socket 1366 mobo will cost maybe $170, with the better ones being more expensive (obviously you've got to make sure that you go for a board with three or six memory slots - an odd number of sticks won't work in a board with four slots), which knocks it up to ~$1000.

Hard-drives are inexpensive, as are cases, fans, mice and keyboards.

You could probably downgrade the graphics card - Crysis 3 may end up on Medium settings when it comes out, but it'll still be perfectly playable.