Depends on your budget. If you get a xeon, you'll have to spend extra on ecc ram, but be very sure you get the right brand and type. Server boards are very specific about the kind of ram they can use. Set a firm budget first, then search for ram and board prices before you make a choice. It's possible to use some server boards with a regular desktop cpu, but it's a waste of money. You're paying extra to use ecc ram for better acuracy, so use the xeon if you get a server board.
Depends on your total budget. But the xeon and ecc ram are the better choice for accuracy. Be sure you get the ram specs for the server board, unless it comes with it. The i7 2600 won't run with registered ram if the board requires it.