Well, Dell charges an arm and a leg for an external PSU for your system, see
here. That will eat most of your budget. A GTX 750Ti is a decent jump over a GTX 645, but it is only considered to be an entry level card these days, it's not going to do high-ultra settings 60FPS at 1080p on modern titles. Sadly, the 750Ti is really your only option unless you want to pay Dell's insane prices for the better power adapter and pray something like a GTX 960 would work, even that might be tight on only 330 Watts.
You might want to seriously consider looking into transplanting your system to a more standard form factor case and getting a regular PSU, that would give you a lot more upgrade options, unless Dell has put some sort of proprietary form factor motherboard in there that can't be transplanted in that way.