If your going to do a DSB(deep sand bed) 4-6" would work well, but you would need add at least an inch of "live sand" on top of any dead sand you've already added. If your just looking for decoration around 1.5-2".
damn... well deep sand bed it is... somebody mentioned to me having an additional sump that contains nothing but sand so as not to have too deep a bed in your display. is this a valid strategy if I want to use a sandbed for filtration as well?
I think I'll be buying some more sand in the near future then. This is quite a project and I violated a cardinal rule of reef keeping I believe. I tallied up what the cost would be to run this thing and that was a mistake. haha