I think, but don't quote me on this, most go with 5/8" wood (mdf or plywood) 1/8" or 1/4" (lexan or plexi), depending on wheather you want a little bit of a lip when you apply the 3/4" t-molding and artwork between.
There should be lots of threads here on this, becouse it's been debated a number of times. Do a search you should find lots of usefull info.
Personally, I used 3/4" Plywood (for strenth, mine is a desktop unit attached to my computer dest), 1/8" Lexan and 1" t-molding with artwork between.