You already have the answer, as it clearly says that the video card cannot draw more than 75W, so you're limits are whatever you can find in a low profile config that uses no more than 75W total.
I want to upgrade the video card on my stock Optiplex 745 SFF
Current Graphics: Integrated Intel Graphics Media Accelerator 965 (Graphics Core: Intel Graphics Media Accelerator 3000)
CPU: Intel Core 2 Duo 6300 @ 1.86GHz
Power Source: 275 Watt D280P-00
PCI: 1 Low Profile
PCI Express x16: 1 Low Profile 75 Watt maximum
OS: Ubuntu 11.04 (could easily be dual booted with XP/Vista/maybe 7)
already know it needs to be a low profile one, I just don't know what
all my options are with the current power supply. I also don't know what
my options are for upgrading my power supply if I need to in order to
support a better graphics card.