Re: would this fans fit on my evga GTX580
The 300 watt thing has nothing to do with the power supply.
The 300 watt rating is just the amount of thermal energy the heatsink is capable of dissipating. In this case, since the 580 is on the list, you should be fine.
I installed a similar unit on my sister's Radeon HD 4870 and it dropped both idle and load temperatures considerably over the stock unit. The brand gets my thumbs up based on that experience as well.