gixx - i'd like to comment on your entry, if you'd hear it, and hopefully the Original Poster will get some use from it too. I'm sure that the OP had intentions of increasing drainage by putting the gravel in the bottom - but here's the thing about doing it that way...It doesn't help. I know this sounds crazy, but it's true.
All soil mixtures have the ability for differing amounts of water to be wicked up into it. The height at which it can lift it, of course, is dependant on the actual minerals, soil particles, and any other ammendment that it is comprised of - as well as their percentage used.
For some mixtures, the capillary action can be so great that is possesses what is referred to as a perched water table. A pwt is nothing more than a mass of water molecules that is suspended in the lower part of the soil mixture and just sits there - where it won't drain out, and consequently promotes root rot of the plant, as well as prohibit oxygen from getting to the root zone.
To the original poster - Since you have sand as the original base, i'd be inclined to remove most of the gravel, put down a good weed barrier on the inside, then fill the even deeper box with a soil mixture that had good drainage characteristics, would allow air to get to the plant roots, and also be able to retain a certain amount of moisture throughout the entire soil mass, from top to bottom. Potting mix (not soil!), perlite, vermiculite, small pieces of pine bark, sand, peat moss, and vermiculite are a few things you can use.
To gixx - Although roots that are allowed to grow down into the native soil below a raised bed certainly provide the ideal growing situation, there's no reason that the OP has to allow it. As long as the soil depth is at least 12-14"and the plants themselves aren't too crowded, excellent results can be achieved by allowing the roots to grow down to the bottom - then sideways. Oh believe me, that's the way my garden is right now, and I wouldn't do it any other way. Besides, the crabgrass below in the native soil is completely overwhelming, otherwise.