My new 24 inch iMac is set to arrive tomorrow (yea!) and I'm wondering what size of battery backup I need? When I was buying a battery for my G5 Quad I kept buying them too small....and I'd like to get it right the first time with my iMac... her specs are: 24 inch 2.8 core 2 extreme 4 gigs ram 750 gb HDD and an occasional external HDD will need to be powered by it as well... I searched for a while but couldn't find the power info I was looking for..... what do you guys think???