I think I actually used that same psu calculator when I was trying to decide whether the q6600 was worth the risk. Unless I left off something (probably a fan), the q6600 + 4650 combo came in with a recommend psu of 258 W and a minimum of 208W. The q6600 (slacr) has a TDP of 95 W and the PSU calculator suggested an additional 27 W (but still below the 280 W rating of the dell power supply).
The caclulator also calculates the 4650 and 7750 as being pretty close to each other (the 7750 needs only 3 more W on the psu). I do think that the 7750 is going to draw less power at idle, and might be better for the life of the psu.
I am probably not going to bother with a graphics upgrade on my sandy bridge box; If I upgraded I was thinking I would try the riser + 7750 on the optiplex 745. If it works, I'll sell off the 4650; if it doesn't, I'll sell off the riser and the 7750. But I'll probably stand pat with my current gpu setups.
I actually am done with the pci or usb wireless idea. I just recently switched to a powerline setup (for my netflix streaming device, not the optiplex) and the powerline I am using has 4 ethernet ports, so I have now just connected it by ethernet. Probably should have just done that all along.