You should be fine. Even though that is a low wattage PSU, Enermax makes excellent quality PSU's. It all about amperage my friend, Not so much wattage. I would much rather have that quallity Enermax PSU than some cheap 600 watt Rosewill or somthing.
I recently built a computer that had a amd 6400+ black edition, a ati 3870, 4 gb of memory, ext alll on a 450 watt corsair psu no problems at all.
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.
it's dual 18A rails with a total of 32A. It will be ok but is definately right at the bottom end of your requirement. You may want to go to a 550w if you plan to do some serious overclocking, but a mild overclock or at stock speeds it should be ok.