i have two sandy bottom tanks now; first one was play sand-- $3.75 for a 50 pound bag from lowes. rinsed it a dozen times and it was clean but still silty when i added water; however running the filter for a day cleared it up pretty quick-- note that it wasn't DIRT in the water, just the too-tiny-to-settle bits.
the second tank actually was already a gravel-bottom tank, but i thought it looked ugly with the different colors of gravel and beach glass bits, so i scooped it out, and carefully added sand-- this time pool filter sand, about $9 for 50 pounds. much less hazy than the play sand, and I got it from a pool supply place near my house. it's whiter sand, and a bit coarser, and i think it's much better than the play sand.
next time i'd remove the fish first though-- but they didn't seem to mind it looking blizzardy for a bit. probably very exciting for them, but it didn't stress them out excessively. the next day they were fine. i think they enjoy being able to root around in the sand a bit too.