Well, when you say dirt, what do you mean? Do you mean native soil that's being brought from another part of the property?
Ideally, in my opinion, you would use about 8" of native soil in the bottom and then top it with about a foot of a lighter planting mix such as 1/3 perlite, 1/3 peat moss and 1/3 compost. That would negate the need for sand, gravel, etc., and the mix will stay light and fluffy pretty much forever. This method has a moderately high startup cost, but I personally think it's worth it and it will last a really long time. I had raised beds filled with the same basic mix at my previous home and they kept going like gangbusters for years. In year 8, the soil was as fluffy and beautiful as it had been on day 1; all I did was add compost pretty regularly and the occasional extra organic amendment for a specific type of plant if I felt it needed it.
And yes, marigolds deter lots of bad bugs as well as some other little critters that might bother your garden. They're always a great choice as an organic pest control in a veg garden! Plus they look nice.

Great, I have a small yard and will be buying whatever I need for fill. I don't mind the expense because 1) It is a relatively small area maybe 5 X 10 and 2) It is becoming a hobby of mine and we love the veggies that I have grown over the past 4 years.

You will be doing your beds alot like mine...
Check out all of my video's to give yourself some ideas..

I use all bulk supplies and mix myself, Decomposed Granite,Pumice,Potting Mix,Steer Manure Compost, Grass and Leaf Compost,UCG's , Blood Meal, Bone Meal, and just about any kind of nutritious amendments I can get my hands on....I used over 80 cubic yards this year...from here on out,I will simply supplement my beds...Whew ! !

Here is a link that might be useful: Raised beds at its finest ;-)

Some of my beds are made of cinder blocks also, but only 1
block high. That's plenty high for the vegetables. Jonhughes,
I like the height of yours. Probably easier to work with, less
bending. I would have a hard time to fill it though. Unfortunately
no leaves fall in my yard, so I cannot make leaf mold. Taras49, I plant
flowers and curled parsley in the cinder block holes. Anything
that is low growing works good. Herbs like oregano, majorum,
chives.

A twent(20) inch high raised bed will need substantial amout of
garden soil to fill it.
Lets do some math: For a 5ft by 10ft, 20" high raised bed you will need " (5)*(10)*(20/12) = 83 cubic-ft
or over 9 cubic yard of soil to fill it.
It is obviously, not economical to fill it with bagged soil, peat moss, pearlit. Those stuff are for potting.
Your best bet is perhaps to find some body/excavator to deliver 9 cubic yard,(combination
of top soil/compost/manure) by truck load.Of course, if there is some decent native soil elswhere in your
property, you can haul and fill the bed with and mix in some horse manure and compost.

Tara, check out the Square Foot Gardening forum and the FAQ's in that section.
http://forums.gardenweb.com/forums/sqfoot/