I'm not a henna user, but from what I know of henna/cassia, it coats the hair, which would tend to reduce your waves.  However, the coating is also known to add body to the hair.

If you do decide to try this - I'd love know how it works out for you.  I quit coloring my hair a few years ago, and have enough grey to be noticable.  Henna would not be an option for me, but I've been interested in cassia to condition my hair and maybe add a bit of golden color to the grey.

Actually henna does not coat the hair, qouted from the henna for hair site "Natural henna does NOT have metals, lead, nor does it coat the hair.  Henna leaves the hair thick, strong and silky!  Only pre made boxes of “henna colors” have these toxic metals that damage your hair". Though the hennatannic acid coats the hair to seal in natural oils, and tighten the cuticles.

Religously i'm not allowed to use chemical dye cuz it coats the hair- water is supposed to reach every single bit of your body when we wash before prayer- so the only other option is henna because hair soaks it up.

and my mom has curly hair and uses henna regularly, and i use it too. So good luck

I have wavy hair and use both henna and senna.  My hair is just as fine and wavy now as when I started using henna last summer.  I personally think my waves look better now.  My hair is much stronger and it is in much better condition.  The color in my avatar picture is my naturally dark blonde hair dyed with a mixture of half henna, half senna in strong chamomile tea with lemon juice for dye release and some olive oil for ease of rinsing.

Henna contains a protein that bonds with your hair, according to what I have read.  I recommend using only the body-art quality henna.  Anything in a box that looks like hairdye runs the risk of containing unlisted ingredients which can damage your hair to the point of complete destruction depending on what they are and what is already on your hair.  

Whatever you decide to use, you should start collecting shed hair from your comb and make tiny batches to test.  This way you can see how your hair looks and feels after henna without danger to the hair on your head.  You said you are starting with the marigold.  Do you mean Rainbow Henna?  I used it a couple times but after more research I would not use it again.  I buy my henna and senna ('blonde henna'...it is a different plant) at the local natural foods co-op in bulk.  

This site http://www.hennaforhair.com/ will give you a ton of information, recipes and advice.  You can also order henna and other herbs, including sample sizes so you can experiment til you get a recipe you like.  I highly recommed Fia's Smooth Gelled Henna, Teabag Method recipe.  I used it and my single cup of senna became a liter and a half of dye.  It is also much easier to get in and out of your hair and does not clog the shower drain.

I absolutely love what henna does for my hair.  It is so shiny that in the sunlight it has an almost metallic gleam.  It is also softer, more conditioned and much much stronger.

Thanks for your post, Sasha.  Your hair looks beautiful.
No, I wasn't interested in Rainbow Research.   I was looking at the Marigold on Morrocco methods site.  I have used several of his products and I'm very pleased(although I know henna is different).
Henna sounds like a lot of work and mess for me but I am interested in the strengthening and general health issues like you mentioned, and I know chemical dyes aren't good. I don't want red and I'm a little intimidated by the mixes on hennaforhair so that's why I want to start with something like marigold first.  I may order her cassia, though, to get a feel for the whole process.
Beesan16- thanks, I was looking at Morrocco's hennas to try first.

Henna provides red coloring.  Using henna on grey hair could result in orange hair.  I would certainly recommend testing it before using it on your head.

If you are interested in the conditioning effects of henna, you might consider cassia.  Although this is not actually henna, it is similar, but has no color.  A strand test would still be advised, but I know of many light blondes that use cassia for conditioning.