A really quick rule of thumb is about two pounds of grain per gallon, with of course variations in efficiency and beer style. But if you're stranded on a deserted island that's what you can go by.
The real question is whether you want to start all grain in 2010 given the lull in the sunspot cycle, possible worldwide crop failures, dogs sleeping with cats, and all the rest. Malt extract has a pretty good shelf life to get you through such trying times, although weird guys with souped-up cars, compound bows (never could explain that one) and feathered headresses will undoubtedy find about your malt extract cache and try to raid it. Your only hope is Mel Gibson. The young crazy one. Not the old crazy one.
Hrm... Or just brew American style light beer. Nobody every risked their life for that swill.