Not to mention the efficiency of the psu will also have a impact on the system as a whole. 25W from a R7 240 is nice, something tells me they will outshine the 750ti when NF gets even higher as you can get them in 4Gb versions compared to the 750ti with only 2Gb.
Spot on about the power supply. I prefer to use the evga supernova 650G (gold efficiency power supply) on my rigs. Another builder used generic grey box PS on his and his efficiency isn't quite as good as mine - I think it may even be as high as 10W per card difference. Do I think I saved the $40-50 difference between to different units in a year? My math tells me it's about $10 per card per year cheaper at $.125 per KWH, so actually, I do. Now, if I am done with a rig for a reason, I have a premium PS left over for some other project or for sale, and it's got fairly low utilization because that rig uses well under 50% of the capacity of that PSU.