Are you building/buying a completely new system or simply upgrading pieces of an existing box? If you're upgrading, there may be multiple issues (that both Dave and Neil bring up) that will influence what you decide upon.
As an FYI, I just purchased a Dell XPS 8300 system with an i7-2600 (3.4gHZ), 16gb RAM, 2TB HD, nVidia GT530 1GB card, wireless N, Win7 Pro, with a 23" LED monitor for $1,140 shipped, taxed and out the door. IMHO (unfortunately), it doesn't make sense any longer to upgrade existing boxes if you include cost. What a sad commentary that we are such a "throw-away" society now.
(As an aside ... the box that this one replaced will be re-purposed as a file server and future additional workstation, so no "landfill additive" here!)
Alan T. Hendry, RA
Intel i7-6700 (3.4GHz) Desktop, 32GB RAM, 3TB HD, Win 10 Pro 64-bit, DC 17, 18, 19, 20 & 21 w/ latest beta update
Intel i7-4702 (2.2GHz) Laptop, 16GB RAM, 1TB HD, Win 10 Pro 64-Bit, with DC 16 and all of the above