Check out:
http://www.garf.org/calculators/SandBedCalculator.asp
Should tell you exactly what you need, and can play with it a bit if you want.
And don't buy "Live Sand" if you're getting any live rock, it will be "Live" after just a day or 2 with rock seeding it. Lots of opinions on this matter though, same as everything in our tanks :P.