Post away my freind..... I guess the determining factor is really going to depend on what you are planning on housing in the tank. And your personal tastes, are you intending on adding macros/plants to your display tank that will require deeper sand to be planted in?

For example, before we took down our 55 long we had a 3-4" deep sandbed, it worked out quite well. Now in the 180 I went half as deep. Not for any particular reason, just looked good to me that way..

Cost savings, we use Southdown sand, if you can find it, it runs around $5/50lb bag at Home Depot stores....it seems this will be become increasining difficult to find, but there are many other choices available, but at a much higher cost.

I like 4" or so in the fuge as it gives any anerobic bacteria a less likely to be disturbed place to live.

Now of course there are benifits for keeping a bare bottom in your display tank as well... Davejnz would be a good resource to tap for that route.

LR, i think the general concensous is around 2 pounds per gallon. Density and/or how porous (equates to total surface area) of the rock can make differences as well.. We started with a bunch of base rock at the bottom and added LR on top of it, now the base rock is very much alive.

Hope this helps some...