I have yet to see my GTX pull as many watts as they claim on that chart Dan. Its old.
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your E5200 will produce a decent amount of points.
GT 240 looks like the only way to go then and probably no CPU. The original 9600 GSO with 96 cores was considered very efficient folding and the GT 240 is a 40nm version of that and looks like it might only use 50W folding. With the rest of the server going idle I think 50W GPU + 50W HDD's and CPU will meet you goal. You can probably sell the 8800gts 320 on Kijiji or the BST forum for close to $50.