Remember that the motherboard will be using a decent amount of power too. My system used 150 W when doing absolutely nothing when it had a i7-920 in it at 3.8 GHz (SpeedStep and C-States all enabled). The X5650 @ 4+ GHz might be a bit lower but not by much I imagine. The Xeon X34x0 CPUs are 4-core versions but also 95 W. The Xeon W3503 and W3505 are 2 core but 130 W TDP. I have no idea if any of these are compatible with any consumer X58 boards though.