Well if you use the stock power supply then the highest you can get is the 750Ti. Don't think it has a PCIe Power connector. If you get an Aftermarket PSU then the only thing stopping you is card length. Just if you do, do that make sure you get some 90 Degree Sata Power Connectors like these
That is if the PSU doesn't have any 90 Degree power connectors on it. You need this for the Hard drive since they plug in next to the case side.
Only other thing is CPU bottle neck. If you have a Core 2 Duo then the CPU would hold back the GPU quite a bit. If you plan on getting a new PC then go for what ever you want and toss it in that later. If you have a good Core 2 Quad then you should be alright but still. I have a Core 2 Quad 2.83 over clocked to 3.5 and have a GTX 480 and i haven't had any issues but then i have yet to play crysis 3 on it yet. I played Crysis 3 on my Core 2 Quad 3.0 GHZ and a GTX 660ti I borrowed from a friend and even on low settings it still laaaagged