TL;DR

It has been about 3 years since I posted this, here is a quick wiki page

http://leagueoflegends.wikia.com/wiki/Takedown

Enjoy

/bump
I would like to see this information as well, not just gold but also silver.
It would be nice to see straightforward numbers instead of having to contunuously compare to other players to get a ballpark range.
From what I can guess though:
Wins:
Bronze-10
Silver- 25
Gold- 100
Takedowns:
Bronze- 100
Silver- 500
Gold- ???
Monster/minion:
Bronze- 2000
Silver- 10000
Gold- ???

These are just guesses based upon my numbers, and the numbers of my friends. It would be great to have an offical word on this though.

I just got 2500 takedowns and it turned gold. I havent checked my profile in awhile but I don't know how many takedowns it used to be while it was silver. I hope this helps narrow the search down.

Answer:

Silver Takedowns: 1,000
Gold Takedowns: 2,500

Silver Minion Kills: 10,000
Gold Minion Kills: 50,000

Silver Wins: 25
Gold Wins: 100
Platinum Wins: 500

well i hit 5k take downs and got a baby blue symbol after gold and i got over 50k minion kills and still gold

Yeah I just hit platinum on take downs at 5k.

100k is platinum for monsters

omg thats a long way to go for me

11031 takedowns and still silver.. **EDIT is actually platinum, I just always thought that was silver :S
89697 minions monsters and gold
370 wins gold