Yep, as long as you have enough watts on the 12 volt rail, then it's good to go. You can try one of the online PSU calculators to see what you need. Enermax is good, so no problems there.
You can probably do fine keeping the older hard drives too. I have 4 in my case with the 3870x2, and my wife (who has a 550 watt Antec Neo) has 3. It's really the GPU, plus maybe overclocking that tests the mettle of a PSU.