I have a gateway fx series computer that i had to upgrade to a new video card. Here were my specs
Intel Q6600 2.4ghz
G33 Intel Motherboard
IDE DVDRW drive dug up from somewhere at gateway
400W Power Supply
I recently needed to upgrade my video card as my old 8800GT died. I upgraded to a Diablotek 450W p/s and an ATI 5770 card.
Running the 5750 on a 300 wouldn't work. It would tax your power supply too much for requirements. Companies put the cheapest power supplies they can into systems to run the components they ship with.
As long as you don't buy the cheapest power supply out there I wouldn't worry about the brand. If you can find one with 80% or better efficiency for a good price you should be fine. But if you're already looking at Newegg, check the reviews.
Sorry, I mean to look for a 450W or better, it can probably run on a 400, but the cost is so minimal if you need to upgrade anyways.