If you have some spare time check out Pete's guide
http://www.subsim.com/radioroom/showthread.php?t=182356
I'd always go above your required amount, but you really need to make sure that your 12v rail(s) is (or are) up to what you'll need Amp-wise. Factor in any further drives etc. that you may include later.
Read that linked thread, it's very informative.
