there is no definite setup for wedding photography.
in my "company" , there are 4 photographers including myself. And i alone shoot using nikon while the rest uses canon. Guess what? Our company motto : Dare to be different.
Well I am different right haha
anyway, for your question, like i said, no definite answers. You can use a d90 no problem. i am personally using d90.
My setup :
Nikon D90 with 18-55vr kit lens( yes laugh all you want)
Sb-600 speedlight
extra 2 batteries for d90, extra 12 batteries for sb-600
4gb SDHC +8gb SDHC backup
Nikon D50 with 18-55 kit lens as backup.
I rent out the Tamron 17-50 once a while .
There are photographers who use a 17-50mm tamron and some using 14-24 as wedding . Some use Full frame for studio shots. My mentor/boss uses 1dmIII for normal shots, while 5D Full Frame for studio shots.
I've no problems with them as I am flexible, I can use both Nikon and Canon. the choice is yours. Is nikon good? Is canon good? My answer ? If nikon or canon isnt good, they wont be in the market right ?what of other brands ? Sure! I saw someone using an Olypus to shoot wedding before no big deal.
To the threadstarter, shooting wedding isnt as easy as one sees...many things can happen.. what i recommend ? Know your equipment and its limits, familiarise yourself and know it by heart then shoot. You and your equipment must be ONE. lastly, get apprenticeship.. some things cant be learnt from books or tips..
D90 ? D300 ? D3X ? 50D? 5DmII ? 1dMIII ? Sb600? Sb900? 430EX? 580EX?? go out, rent out , shoot more then decide.