With a $500 maximum budget, getting a 750 Watt PSU, GTX 750 Ti (or GTX 960) is a HUGE waste.
If you got the 750 Watt PSU, then get a real GPU and not a pathetically weak one like the GTX 750 Ti.
The GTX 750 Ti didn't require you a new PSU purchase so it's a waste to get a new PSU for a GTX 750 Ti.
Either A)
Get ONLY the GTX 750 Ti. R9 300 series and the GTX 980 Ti are coming. Along with next generation's 16 nm GPUs. If you get only the GTX 750 Ti, you save $400 and have that for when massive perf leaps come.
B) Get a GTX 970 or R9 290x and a 750 Watt PSU. The R9 290x with a 750 Watt PSU would be around $400 which is $100 under your budget and is amazing perf.