(Redirected) Alienware x51 R2 GPU Upgrade

I have been looking all over the internet for a solution to this question but have yet to come across an answer. My question is whether or not I could upgrade my current default GPU and Power Supply, to better GPU (NVIDIA GTX 780 3GB for example) and a 750 Watt Power Supply (EVGA SuperNOVA 750 B2 80+ BRONZE for example). Many of the computer games coming out nowadays can barely run on the default GPU on the lowest possible settings, and changing the GPU (which is not really a problem) means getting a new power supply (which is the problem) is truly the only option, other than buying a computer than can handle the stress of these games, or building one from scratch.