I'll give it a shot
AMD 4000+ = 65W
Intel Q6600 = 105W
Intel Q9540 = 95W
Mobo NB + SB = 30W x3 = 90W
ATI HD3870 = 106W x2 = 212W (@496GFLOPs ea. = 4.68Gflops/W)
HDD = 20W x3 = 60W
Optical Drive = 5W x3 = 15W
1 stick RAM = 10W x 6? = 60W
80mm fan = 2W x3 = 6W
120mm fan = 2W x3 = 6W
TOTAL for 3 rigs: 714W x 1.2 (for Min. inefficiency of PSU) = 857W pulled from wall.
Total processing power (CPU + GPU) = 66.5 + 992 = 1058.5 GFLOPs
Your current combined Efficiency: 1.24GFLOPs /Watt
*** *** *** *** ***
Sandy Bridge i7-2600K Sandy Bridge = 95W
mobo NB + SB = 30W
HDD = 20W
Optical Drive = 5W
2 sticks RAM = 20W
80mm fan = 2W
120mm fan = 2W
ATI HD5870 = 188W (@2720GFLOPs = 14.5GFLOPs/W!!!)
TOTAL for new rig: 362W x 1.2 = 434W pulled from wall.
Total processing power (CPU + GPU) = 83.3 + 2720 = 2803.3 GFLOPs
Your NEW combined Efficiency: 6.46GFLOPs /Watt
I think that might be enough to convince her - new rig is 5.2 x MORE efficient than running the 3 old rigs put together. In other words, it has 2.6x the processing power, and uses exactly half of the electricity to do it.
Hope that helps!