300 watts is the minimum recommended PSU for the GTX 750 TI, it will be fine. The regular GTX750 TI is only a 60 watt card. The OC isn't going to make it draw that much more, generally 10%.
My emulator box is running a GTX 750 TI / G3258, and it's max draw during 3DMark11 is 114 watts (from the wall).