Kyanar: I still highly recommend a modular power supply if possible - that way you don't have so much cable mess permeating the case (should get you slightly better airflow too!)
550 will be fine as Krishant says for that spec - but if you expect that you'll be adding much more later (e.g. USB peripherals, extra graphics cards, more than a few fans) you might want to move up to a 600W to ensure you have some buffer room.
Remember- USB devices count towards the total (they draw a couple of watts max), as do the internal fans (usually 1.5-2W each, more if it's a fancy Cooler Master type fan with a control panel and LED and suchlike).
Also, in my experience, Gigabyte is terrible for motherboards. After RMAing three Gigabyte motherboards in a row I swore I'd never by their mobos again.
I agree with the modular power supply bit. The less wires you have in your systems, the better the airflow and heat dissipation.
And yes its true about the fans etc. They will all chew power. And generally it is better to be overpowered than to be underpowered.
But that doesnt mean go get 1000W power supply. Go to the bear minimum of what you need and increase it by 50% or less to be safe i reckon.
I used to have a 350W power supply in my last machine that used to power a Quad core Q6600 processor, a Nvidia 8800GT gpu, 8 USB ports, Wireless card, harddrive and DVD drive. That was a Dell build and not a custom build btw.