The sales guy is going off AMD's recommendations for a power supply. AMD and Nvidia both overestimate the power supply requirements because there are a lot of low quality PSUs out there that claim they can do 300 Watts, but really can't. The stock PSUs in most OEM machines are okay, and will deliver the wattage they state, so as long as it is a 300 Watt or higher power supply the 7750 will run fine. If you want to step up to a higher powered card though, you would have to get a bigger power supply.
Okay I see the general answer here is that the 7750 will run fine. But is HP known for making quality 300w's or not?
This a newer computer though, so it shouldn't be too bad.