If you're switching to sand just for the look, then go as deep or as shallow as you like. A 2"-3" covering will give some nitrification benefit. The bagged stuff you buy at the LFS
might have some bacteria in it, but you don't need it. It's not worth the money charged. Find caribbean play sand in the garden section of your HD
or get silica playsand (the stuff meant for kids sandbox).
Do you have any live rock? Are you planning on adding any? LR
will seed the playsand. Detrivore kits are available from several online places or if you can get 'borrow' a cup of sand from one or more established aquariums you'll have plenty.
What's in the tank now?
What are you using for filtration?
How big is the tank.
There's a and depth calculator on this site that will give yo an estimate of how many pounds of sand you need to reach a given depth.