I have a Rosewill Stallion 400 watt power supply, and it has two 12 volt rails: one with 13 amps and one with 14. Will this be enough to power a GTX 750 Ti, which requires 20 amps? The PSU has one PCIe power connector. I also want to know in general if multiple 12v rails "stack" amperage (i.e: does this PSU have the equivalent of a 27 amp single rail?)
Yes, the GTX 750 ti uses the 6 pin PCIe connector. It has two yellow wires, each of those wires is one of the 12v rails. When you connect power connections in parallel you get to add the amperage together so it should be the equivalent of a 27 amp single rail.