GTX 750, non Ti is still faster than HD7750, and the cheapest one on Newegg right now is 89.99 after 20.00 rebate. Or you can get a 750Ti for 119.99 after rebate.
Cheapest HD7750 is only 10.00 less than the GTX750, but doesnt involve a rebate.
I assumed only the XPS dells with the 460 watt psu had six pin connectors (I have one, it has 2 actually), but you can check on the 350 watt. If it has a six pin connector, you could move up to something like the R7 260/260x although that might be pushing it. Performance will be close to 750Ti but about 30.00 cheaper.
Edit: how old is the computer and what is the cpu? If it is an i3 you will have a bit more power budget to work with, but personally, I would just go with the 750Ti. Maybe not the absolute best performance per dollar, but it has 2gb ram and for sure should not overload a 350 watt PSU of decent quality, which I think dells are. The 750 Ti was seriously overpriced on launch, but it has come down to decent levels now.