What is your style of shooting and what kind of things you would like to shoot in a wedding?

Journalistic? Portraiture? Group Portrait?

jz get d kitlens 18-55 1st, its good in everything but not best in anything
wat kind of shots u want that ur kitlens cant take in wedding?
do u need tele lens for wedding? then get 55-300, but be warned for poor low light performance, unless u get 70-200 f/2.8 which costs 6k+
do u need 50mm? or u jz considering cos its cheap? but be warned to be frustrated by manual focus
might as well get an external flash which is considered to be a definite must have for wedding

i recommend flash cos it has universal usage which helps whichever lens u r using, its useful oso even outdoor during daytime 
as for 50mm, if u r shooting group portrait or couple portrait where they r going to pose nicely for u, then manual focus wont b a problem, but for candid shots u may miss some good moments
20-200 will b useful as all-in-1 lens, u can take group pics with wide angle n zoom in for candid moments oso

ur choice 

i dont think there is any 20-200mm

got 28-300mm
18-200mm

get D7000 instead lol.

but why you wan sell D3100?

That's a price for brand new unit. 

But spec wise, you're consider as downgrading. Better get D7000 instead 

Ohhh! You didn't mention with kit, so I assume D90 body only. Woot, that's cheap! 
Items 100% in great condition? Sounds fishy 

use the money to buy lens instead of upgrade body ~

This lens is prone to hunting during lowlight, if can spend more, just buy the Sigma 17-50 OS ( About RM2400 Grey Set )

i use d tamron vc for wedding before...
if u hav a speedlight...
den everything should be ok...

Means vibration reduction, prevent blur picture from ur shaky hand

D3100 doesn't build-in motor and VR, all depend on lens

How about save it for the upcoming D800? And that gonna be like a Bugatti perhaps? 
Wait a minute, are you trying to say D3100 quality is bad? Clarify further please?

@Irfan How do you know the quality of the D7000 sample pictures were damn nice?

Camera screens are so deceiving laaa. What looks nice on it, won't be so nice on your computer monitor.

Anyway, you should have tried some lenses over there *envy*

if d7k is ferari, the d700 is wat? bugatti veyron?