The 745 was born in the days before 80+ certifications really made it into the business desktop market - for instance even the 9020 today doesn't have an 80+ rating stamped on it...
You're sort of limited for upgrades by design, Dell didn't want people changing too much so they put in a totally non-standard power unit, I think they used to call it BTX or something?
I don't think any low-profile cards come needing PCI rails on the PSU, because the manufacturers know you'll be sticking it into a dell or HP or some other small form factor unit. Besides, the small PCBs don't really leave room for proper voltage regulators and other MOSFET stuff that comes along with the extra power.
IF they will fit in your case (the 745 series was actually a few different PCs, they changed a bit over time, different internal designs), the following are about the most powerful low-profile consumer-market GPUs I can find:-
NVIDIA Option: Gigabyte NVIDIA GTX 750 Ooverclocked Graphics Card - 2GB - GV-N750OC-2GL - Scan.co.uk
AMD Option: XFX Radeon R7 250 Graphics Card - Boost - R7-250A-CLF4 - Scan.co.uk
the AMD option doesn't display it's TDP, but the NVIDIA option says 55W. AMD tends to be a bit higher so I'm going to guess 60-65W. You should be fine with the stock PSU, which i think is somewhere between 300-350W, possibly 400W.
I had a Quadro NVS315 working in a 745 not too long ago, so I think either of those would be fine. The 315 has a TDP of like 20W, so I don't think an extra 30 watts on the PCI lane will be a problem