Hey bud, if you want an idea of what 150 lbs looks like in a 6' x 2' tank, here ya go. I used 150 lbs in my 225 gallon but it is a 30" tall tank.
I think it's a bit much but I have geophagus who love it and plecos that love to dig. I think 100 lbs would have been perfect. It all depends on your taste, stocking and maintanence schedule. My sand gets stirred up once a week during WC's. Hope that helps...
I don't think it really matters on how many gallon's your tank holds, it's really all about the footprint. You're probably going to have to see as you go, if it's a cheaper sand like Pool Filter or Play Sand, it's probably best if you buy more than enough, versus a more expensive option like sand from your LFS.