totally depends on what you are going for. you can use anything from none (ie. zero pounds) to a 6+ inch sandbed which will be a couple hundred pounds.
do you want it for decoration or do you want it to try to do a dsb? figure out what you want to do first, then figure out how much you will need.
ok so i just want it to be decorative so 1-2 inches will be fine. what type cause i see so many. i got some base rock from bulkreef.com and done my aquascaping will do another 25 lbs of live to seed.
sugar size is nice but it blows everywhere; crush shells or larger grain sand tends to collect and trap ditritus and most inverts and fish dont like it.
this is a basic math ex. to find out how much sand you want or need for your tank.
tank size 48" L by 18" W u want 2" of sand: 48x24x2= 2304 now devide that by 14 =164 lbs of sand. white sand is nice cause it reflects the light back up
I went w/ a 2" sand bed on a the same dimension 150, wish I had gone 3. Remember that w a tall tank like that you are you to be trying to push some flow downwards so I would stay away from sugar sized. Here is a pic of my old 150 w/ the 2" bed that originally was a bit over the bottom trim.
Also consider what you want to keep in there. Some livestock refer deeper sand than 1 or 2 inches. You can always add more or take some away slowly if you don't feel happy.
I think the bigger question is grain size.