We have a couple of the Optiplex 755s here at work. I measured the power of one of ours here with a C2D E6750, 4x1GB, and a Radeon X1300 to be about 62WAC idle and 103WAC under heavy CPU usage. The 7750 runs off the PCIe slot directly (no external power needed) so the max is 75W. The 150W estimate probably pretty accurate.
The Dell PSUs in these are just about standard ATX. Connectors are all standard. Only difference is that they are slightly taller. Mounting holes are the same even. So you would have no problem dropping a standard ATX PSU in there. You would actually end up with a little bit of extra space between the PSU and motherboard. Might help with cable management.
You can use these Dell PSUs in standard PCs. Assuming you have the extra room around the PSU for the added height. I have one of their 80+ silver models in an Antec mini-P180 case. Had to cut up the case a little to fit it in though. And since the 24-pin cable is so short, an extension is pretty much required.
Did you have any problems with the SATA power contectors? I'm told that if they're not right angle it might cause issues? Antec VP450 doesn't have right angle SATA power contectors but most budget PSU don't. I see in most of the reviews the HD 7750 drawing about 47 watts, I use them in few my systems since I only game 1366 x 768-1440 x 900, quiet and cheap. Great, if I can get into the case without making a mess it would be nice, maybe I'll try after all, Thank you for the information.