The Xeon 3500 chips are based off the Nehalem architecture, Xeon's are workstation processors too. The equilivent desktop part would be an i7 9xx.
The Core 2 Extreme is the top of the range desktop part, the Ark website does list them as disscontinuted.
The 3500 depending on clock speed should be faster than the C2E.
As the C2E isn't available any it's hard to say what the prices would be.