L4D is a hard game to learn correct movement with because it will almost always black dot eventually, but at the same time when it black dots it gets a huge kick of acceleration. Find a sensitivity that doesn't black dot when you move your mouse at 50% speed. That should be about when the acceleration levels start to change in that game.
Yes using a mouse with at least 1600 dpi would improve your performance, wired couldn't hurt either. The higher DPI the better. Check out the wireless mice I've recommended in my previous post.
Another issue is you're using a beta translator, it's altered from the original and it is known to start out slow. There was a general consensus that the previous method used was better and hopefully soon we should see that translator revert to the original. Portal 2 is another game that's running a beta translator with the same issue that will hopefully see a revert as well.