What should I upgrade next?

Hello Chaos21,
You don't need to upgrade the PSU with that videocard and I'm sorry to tell you this, but you won't be able to play those games w/o lowering the settings down, even the resolution.

Well, I installed it and I had to modify my case to fit the card in, and I had to keep the side of the case off. It's been working fine, I run America's Army 3 on highest settings with no lag, garrys mod on all high almost maxed out ( I think AA was off ) With 90 fps. I'm scared to transplant cases, and I don't know what power supply would be compatible. Need more feedback.
Thanks,
Chaos

EDIT: Oh and by the way I have very low resolution, I think 1024 x 768, so it won't be too much of a problem. I don't have money so I probably won't upgrade the monitor soon, PSU first, then maybe cpu..

wait until you try games like starcraft 2 and battlefield 3 your video card wont be doing so well...
the 8400 isnt that energy consuming anyway and your psu would be fine, try running furmark and if your computer dont crash then there is no need to upgrade your psu until you add something more powerful on e.g. when you upgrade your processor or into a higher end card. also you can save that money for that little bit extra in the future if you need

I can play starcraft II on all medium with no lag  . Still no suggestion for PSU? Or transplanting my computer to another case? Because I do want to upgrade CPU in the future or my card.

well you dont need to upgrade your psu at the moment, save the money and when you get the money to upgrade to something that does need a better psu, use the money to buy a suitable psu then. otherwise there is no point doing it now