So, go to new egg, look at CPU's, and look at thermal design power in the advanced options:
So, I start with the same 140W the two Xeons are rated at, and come up with AMD Phenom X 4 9550 Black Edition. Not bad price, 157. 2152 on passmark, but, figure you buy that to overclock, which means eating more power.
Back to the drawing board.
Lets try one cpu, at 73W.
290. Intel Core i5-670 Clarkdale 3.46GHz 4MB L3 Cache LGA 1156 73W Dual-Core Desktop Processor BX80616I5670
passmark:
Intel Core i5 670 @ 3.47GHz 3163 113
Better numbers:Intel Core i3-550 Clarkdale 3.2GHz 4MB L3 Cache LGA 1156 73W Dual-Core Desktop Processor BX80616I3550
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115065
i3 550 for 150.00
Passmark
Intel Core i3 550 @ 3.20GHz 3393 98
Also at 73W
That's kind of the search pattern. I look at the W on the processor, figure it has to be at least half to be worth doing, and, the performance should future proof the machine a bit, so I figure 3 times the speed, minimum, of the dual setup, which is about 1000 on Passmark.
The i7 870, @ passmark Intel Core i7 870 @ 2.93GHz 5866 and 95W looks pretty good, except for the price, 580.
The i5 750 is tempting Passmark:Intel Core i5 750 @ 2.67GHz 4202
95W 200 dollars.
AMD Phenom II X4 945 3588, fast, 95W 140.00.
With a lot of research, the Black Edition X4's look like they might be an alternative, since they appear very reasonably priced, and, if they can be overclocked without messing with other settings, for 100 bucks you can get into top 50 speeds.
From there it jumps pretty severely to 125W, which while increasing speed a lot, doesn't do much for power consumption.
That's what I've been doing to look at this, and, that doesn't even get to sockets, ram, and motherboards.
My end conclusion is for me, I don't see a cheap upgrade that would justify the cash outlay. YMMV.