The dell OEM PSUs are pretty poor quality from experience, if it is a generic Dell OEM one that is (which it probably is).
Well, depends on how much power is going through the 12V rail, you should be able to see a label on the side of the PSu stipulating the W output on each rail. It's only the 12V rail that powers your GPU, there's a slight misconception that it's all about overall wattage, when technically only the 12V is what powers your major components.
In short, ideally you'd probably want to upgrade your PSU, as the Dell OEM ones are usually very poor quality.
But i suspect it will probably be fine for a while, you may just want to take it as a caution.
The 750 Ti is a very low power usage card for the performance, but regardless it's not ideal to pair any modern GPU with a poor quality PSU.