Replying to Canon 70-200's ?

So, i'm asking that question here on NS because I feel that people are more close to what I'm doing, which is shoot snow, skiing and others.

I've got a 7D for quite a while now, bought à Tamron 17-50 and a tokina 11-16, which I'm really happy with. When I bought it, I got the 18-135 which I know isn't the best zoom lens out there but I just wanted something to begin, plus I didn't pay extra (would have gotten the body for the same price with or without it). Anyways, I've came to the point where I really see the weakness of this lens, and I'm tired of using it.

I searched a bit on the interwebz, and came to the conclusion that I probably wanted a 70-200 because of the fixed aperture and the nice zoom range. But there are many 70-200 models (IS, non IS, USM, f/4, f/2.8 etc) and I'm a bit confused. I can't decide, of course I would love to buy the f/2.8 II, but it's so expensive and I can't really afford it now.

So basically, I wanted to ask you about your personal experiences with any of those lenses, the + and - of any type etc. Everything you know about it. I really need to make a choice, but I'm pretty limited because of the money. And if you got any other alternative, an other lens. I'm really looking for something sharp, with nice contrasts and it must have a fixed aperture. Weight and color (yeah we never know) doesn't matter to me! What would you recommend me?

Hope you understood what I'm trying to say! Thanks in advance