1k on an i7 990 unlocked.
500+ on a gtx580-590, maybe two.
200-300 on a case.
200-300 on a mobo.
1k-1.2k watt PSU.
An SSD or two.
Or just buy that $1500 limited edition GTX card with two 580s built into it.
And then congratulations, you have what in 2-3 years will be considered a budget gaming computer.