The GTX 750 has such low power requirements it does not even have a power connector, simply drawing what it needs directly from the PCI-E slot. Even on a 300w power supply you will be perfectly fine with adding a GTX 750 to your system.
However, I highly recommend you spend an extra $10 or so to instead get the 2GB version. The 1GB of extra VRAM will help quite a lot with many games.
Infact, looking on the Newegg site, They actually have a refurbished GTX 750Ti for $115. This card would still drop in and play, but has 25% higher performance and 2GB of vRAM. Refurbished parts still maintain a warranty, so if by chance there is still an issue with the card you can still freely return it for a replacement. (Cards are...