Get an i7 2600K or 2700K and Z68 or Z77 chipset motherboard, so you can overclock it, off eBay. You'll may have enough left for another 8GB RAM. Avoid OEM boards like Dell or HP. Get something like a Gigabyte, ASUS, ASRock or MSI.
You can also try to find a combo. It will be harder to find but offer less hassle. Since you get a full unit. If something doesn't work you send it all back. While buying separately can sometimes net a better deal. You also have to figure out which is bad if you get a bum part.
You can also buy a used Dell, HP, &c. With a 2nd or 3rd generation i7. Just be very careful of the design layout. Some use a useless BTX layout. Others have proprietary PSU connectors.
Just be aware with eBay sales. Preferably...