Planning questions for 40 gal tank

You'll want live rock to seed the tank, just don't get the expensive decorator kind. There are lots of sources--Tampa Bay Saltwater, www.ipsf.com, Foster and Smith--do a search for live rock online, and you'll find several threads with sources. Supplement this with tufa, and you're good to go.

I'm gussing you asking about how to aquascape and have something decorative in the middle level of the tank? Stack the live rock--you don't want it covering all the substrate, and piling it into towers and such forms caves the fish will use.

I like powerheads--makes it easier to direct the water where it needs to be moving. If you use a HOB, run it without any media, but don't count on it providing adequate current for the entire tank. Heck, I have 2 powerheads in my FW 40--HOBs just leave to many dead areas, IMO. The live rock and sand is your filtration, and using conventional filtration means more work for you to keep nitrates down.

my usual plug

If you want to give yourself an advantage in figuring all this stuff out, Bob Fenner's Conscientious Marine Aquarist and John Tullock's Natural Reef Aquariums are very, very helpful. There are a lot of books out there, but most are outdated, these are great. Especially nice if people are asking "what do you want for Christmas?"