1sttimepcbuilder :
Well my friend who has a masters in electrical engineering and builds pcs says i should go with a 750w or higher so it doesnt get as hot and will last longer
All the PSU experts on jonnyguru.com would have to completely disagree with him, and they are PSU reviewers and experts from around the world. There's a few reasons not to do that.
1) It will be less efficient. He is incorrect about it not getting as hot. Waste heat is directly related to efficiency, and in your case a 300W power supply will have a lot less waste heat under the same load as a 750W one, meaning the internals of the 750W one will be hotter.
2) There's no reason to spend all that extra money, and it is a lot of money, for a 750W unit when you can get a 450W unit that does just as well.
3) Theoretically it may last longer. It depends on a
lot of factors, though, it's not a simple matter of higher wattage PSU = longer lifespan. As I already mentioned, more waste heat will actually lessen the lifespan of the capacitors in a power supply, so really the lower wattage models will probably have longer living capacitors. Of course, it all depends on other factors; higher end models may use more capacitors, meaning they deal with less ripple and therefore may last longer, but a generalized statement like that cannot be made. Even if it does last longer, we may be talking a lot of years vs a lot of years. Not really worth the price justification.
In the end, you spend a lot more money and end up even losing out on the energy bill since PSUs are generally most effient at 50% of their rated wattage. He may have a Masters in electrical engineering, but so do some of the dozens of experts on Jonnyguru.com who all disagree. It's just a lot of money for one.
Here's a better picture: it's better to get a high quality 450W unit than a crappy quality 850W unit.