X51 GPU Upgrades (Intel(R) Core(TM) i3-2120 CPU @ 3.30GHz, 240W PSU, NVIDIA GeForce GT 545 R1 Model)

So, the time has come where I feel that I cannot play games to a high enough frame rate anymore and I feel an upgrade is required. However, I am aware that this may be quite a task due to the 240W PSU. If im honest... I was hoping to go for a card like the GTX 960. Something that will last me a few years while not being extremely dear. I do expect to play games at a graphics setting that looks nice while maintaining a good frame rate. I dont play extremely graphically demanding games however; the most demanding game I play is probably DayZ, and we all know that game is so demanding purely due to poor optimization.

While im definitely not a PC know-it-all, I know a fair bit about the internals of the system as a result of dismantling it and reassembling over the past few years, however I have one issue/question. In order to upgrade to one of these bigger cards, such as the 960, would I need to purchase a new PSU, such as the 330W unit? and if so, do I have to make any modifications internally, or is it as simple as just plugging in the 330W adapter? Would I even need to purchase this 330W unit, as the GT454 I currently have installed has a max required wattage of 105, whereas the 96 has a max required wattage of 120?

Thanks for taking the time to read my post. Answers are appreciated.