If your PSU is not good enough for your video card, it can cause freezing/crashing yes.
The DELL cases often have proprietary PSU mounts which means to you that you can't be sure any non-dell PSU will fit in the case without you having to modify the case.
That being said, a Corsair CX 430w should do the trick fine, or an XFX 450w.
There isn't really a max power you should go to as much as the fact that you really want quality more than power, which rules out about 90% of all possible PSUs (the ones that are complete garbage).
The two I mentioned, though, are most definitely not garbage.