I'm sure you've already made the purchase of the X5690, but if not, take a look at the X5680. It's roughly half the price of the X5690 (roughly, and you might find a rare good deal on a X5690), but is just a teeny bit slower. About 3-4% slower. You won't notice it, and you can save money.
Also get the X-series over the W-series (again, unless you find a rare ultra-low price on a W chip) because you never know if you might get a dual CPU tray or dual CPU MP in the future, and you can use your existing X-series and would only need to buy one more. The W series cannot be used in a dual config.
Have fun!