You can run any card you want to. The two things to think about are what version of pci express you have (2 or 3) and at what point your cpu will become your limiting factor, as opposed to the bottleneck being gpu, hard-drive etc.
With respect the pci express, in my own opinion, the differences are not massive, but are there. This is determined by your mobo and what version it supports.
With the above in mind, you can then determine price-range and thus gpu you're after.
Personally, I would advocate you buy a bomb gpu because it's likely you will run with it even after your next upgrade cycle of mobo/cpu. So be it if you notice your gpu performs better with new mobo and cpu, that's only a plus.
I can see if the guy I got my card from still can get 780ti's for $520 cash, if you want.