This is a very widespread suggestion. I've yet to use it but am actually taking a break right now from breaking down an existing 55 Gal to redecorate it including changing it over to sand. I'm going to price black sand and if it's too expensive I'm getting pool sand. I was advised (a friend manages a pool store) that $10 per 50 lb is a good price to pay, anything more is excessive.
As far as filtering... I always promote the usage of Overflow filters or canister filters. I know Whisper makes a micro over flow filter that actually sits inside the tank. This is what I have on my 5 Gal betta tank. Works fine. I discourage the use of under gravel filters for several reasons. I think a 5 Gal is probably too small for any kind of canister filter that I'm familiar with. So it looks like a very small overflow filter would be your best bet (in my opinion). If it has an inlet tube (the one that sucks water in) make sure it isn't too close to the bottom of the tank that is sucks up sand when it gets stirred around.