Just buy the parts.
Lose the i7 and go with an i5 from this or the previous generation (i5-3750k, 2500k, 2550k)
8GB RAM (DDR3, 1600 speed, 1.5v)
A Z77 motherboard. I own a Gigabyte GA-Z77-D3H
Case with USB 3.0 ports up front
128GB solid state drive. Intel 520, Samsung 830, Crucial M4 all good choices. (You can get a bigger solid state or add another mechanical drive if you need more space).
A Hyper 212 Plus CPU fan
Quality power supply - depends on video card
Windows 7 Home Premium 64-bit OEM
Then find yourself a video card with some help with from this article
. Read reviews of the different brands and price shop obviously.
You'd be at $800 before the video card, and I'm overbuilding everything and estimating high. Tons of room to scale back without that big a performance hit to worry about. If you don't care about USB 3.0 on the front of your machine or want to go with a less expensive quad core you have options that will more than meet your needs.