And keep in mind that 750 watts from a "cheaper" model tends to be very different "watts" than from a high end model. Engineers design them. Marketing puts the labels on the box. Usually, the difference is the cheaper model can make it's rated power if everything is PERFECT. The high end model can make it's rated power when everything is hosed.
Also, there are more than one power rail in a power supply and the sum up ALL the power rails to get the supplies rated power. That can very much be the difference between models that work and models that don't simply based on the split of power between the different rails (aka: not enough juice in the +12V rail).
And, when it comes time to invest in a new supply, keep one thing in mind: They tend to hang around a LOT longer than many other components in the computer do, especially if you buy wisely up front. So be willing to spend a bit more on a better supply. It is cheaper in the long term.
I had a friend many years ago who wanted to completely upgrade his computer, but it wanted to keep the old case and power supply to save a few bucks. The PS was a VERY cheap POS supply that he had picked up in the past because it was cheep... I told him there was no way in HADES I would reuse that supply with newer/better components.
He went cheap.
Two months later he called me over to trouble shoot why his computer wouldn't turn on.
The power supply had failed with the increased load. And when it died, it killed his mother board and CPU as well.
Don't be cheap on power supplies.