The tricky thing here is the 290x is a power hungry card. It's is going to use a lot of power under load, do not underestimate that. The FX 4300 uses another 95W in addition to the card, both from the 12V output. Not only that, but you don't want your power supply running at anywhere near full load all the time, you want some head room. So I would take that 750W recommendation seriously with this particular CPU/GPU combo.