We've tried this a couple times. The result is never really the same as Jamaican ginger beer, but worth experimenting with. We've added much more ginger, like 1-2 lbs for a 5 gallon batch. There is an interesting interaction between malt and ginger, as in, the malt completely hides the ginger flavor until its aged for about 9 months. The first time, we boiled the ginger for 10 minutes and after fermentation couldn't taste any ginger. So we added another pound of chopped ginger and aged it for a couple weeks. Still no ginger. After it was in the bottle for almost a year we could finally taste the ginger. Beats me what's going on - I'd like to know how you make out.
Other things. Someone else made a ginger "beer" once that was actually a ginger mead made with 50% honey/50% rice syrup. It was really good.
The second batch I used a blend of spices with the ginger - lemon, orange peel, and cayanne pepper. I decided to do this after reading the ingredients on a bottle of british ginger beer that listed "blend of botanicals" and capsaisin (hot pepper oil). Be careful with cayanne, but it definately helped give it the "bite" that I was looking for.