Definitely dump the UGF. I ran one for a while before I switched from CC to DSB. They are OK at best when used with CC, but will not be beneficial (may actually be harmful) to a DSB. I can't comment on the Fluval... never used one.
I have a 44g tank and I have about 80lbs LS. However, mine is a pentagon tank so its footprint is probably smaller than your standard 40g tank. Just guessing here... I'd say that you'll probably need approx 120lbs LS. Give me the dimensions of your tank and I'll be able to give you a better number. For comparison's sake, my 75g tank (48"x18") took about 180lbs of sand.
As for a skimmer... I don't use one. I believe that a balanced tank that is well taken care of (periodic water changes, not overfed, not overstocked, good amount of LR/LS) doesn't need one. That's just me though. The need for a skimmer has been discussed several times on this board in great lenght... there is no right answer on this one. However... having one will never hurt you, but not having one might.
HTH