Can my [custom] computer be powered by a 750 watt power supplies system?
A power supply won’t fry your system just because it has more capacity than you need. The main drawbacks to getting a power supply that is way more powerful than you need are that more powerful PSUs cost more, and it might run less efficiently at loads that are very low compared to its maximum capacity. A good-quality 750w power supply is plenty for that system with one 4870. It would have enough capacity if you decided to get another 4870 and do CrossFire, though there may be issues with some supplies regarding having the right number/type of connectors and amps/rail. Just make sure your PSU is a good one (e.g. a Corsair 750tx or some other quality model).
