There is a PSU calculator here, you fill in the blanks for all your components, it gives you an estimate of the Watts. I would suggest Corsair's 750, either the CMPSU-750TX, it's about $115 today. Or the CMPSU-750HX, it's about $150 today and has modular cables - you only use those you need, it helps with cable management.
Trifire doesnt scale quite as well as crossfire, so depending on how much 2 4890s costs you might want to get a 5870 or GTX 480 which will perform a little slower but gives you the option to add a second in the future.
Rather than get a cheap basic 500GB HDD, grab a samsung F3, its not too much and is a very faster HDD especially for the price, you will notice the difference in load times.