EpicTaco5 :
Well by fit I just mean fit in my case. Also, a 350W psu would be enough for this, right?
Oh, right - well I've just looked your computer model up on Google Images and the answer is YES, it will be properly handled by your case.
CPUs are mostly equally big - physically speaking. From a temperature standpoint, your case should be able to handle the Phenom II X4 955 without a problem, especially since it also has side air intakes.
I would need some info about your graphics card. You said you've just upgraded it, so I'd need to know the manufacturer and the model of your graphics chip.
With a card like the HD7750 you would have no problem upgrading to the Phenom II X4 955 WITHOUT also upgrading the PSU.
Heck, I think even the PSU should handle even the HD7850 together with the 955 Black Edition rev.C3.