Help with Boeing 737-800 NG

I am looking to download a free Boeing 737-800 NG for fs2004, but am not sure which one to get. I want to get the best, most popular model so I can download repaints for it in the future.

While browsing on avsim, I have seen some by a company called 'Kittyhawk' and some by another company called 'aiaardvark' but I cant find an offical page for 'Kittyhawk' and 'aiaardvark' planes appear to be AI aircraft, so I'm guessing they aren't good quality or accurate to fly???

Could anyone please tell me which is best, and why?

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Also, while browsing down this page 'http://www.ai-aardvark.com/aiaircraft/aiaircraft.html' I have seen there is such thing as a B737-900. Is that an 'NG' plane or is it older, because I have never heard of it before.

Plus, how come the 737-500 is smaller than the 737-400? Whats the deal with Boeing adding 100, 200, 300, 400, 500, 600, 700, 800 and 900 extensions to thier 737, 747, 757, 767 and 777 models? They don't seem to be in any order of size or anything, so whats the deal with that?

Cheers