Well if it's choppy, you're going to need more than just a better Power Supply, you'll need a better GPU.
Looks like you have a PCI-E 16x slot, but I'm not sure which version (1.0 or 2.0). Assuming it's 1.0, don't get any PCI-E 2.1 video cards. If it is in fact a 2.0 slot, you'll be fine.
You might want to post what your CPU, RAM, Monitor size and resolution, in game settings you want to play at, and what your budget is.