I guess what mouse24 said is really relative, I had a different experience with mice.
I first had this crappy $20 optical gigabyte mouse and it did pretty good for what I was doing, I was able to aim pretty decently but as soon as I flick shot in the slightest it added lots of acceleration.
late on I got the Razer Imperator, It fixed most of the acceleration (this is where I started getting into competitive gaming) but it was really noticeable. Took a break from the fps scene and the Imperator gave out because of a common flaw.
I then went to a Razer naga because I was into mmo's and didn't play much of anything else, but with the release of cataclysm I couldn't play wow anymore after all the years I had in the game so I went back to competitive fps gaming. I soon then realized that the naga was not a fps mouse and I had to sell it and then I finally got a mouse that could keep up with my reaction flicks. The zowie EC1evo has a almost perfect sensor. I noticed 0 mouse acceleration at high speeds with it so I was able to make every shot accurate.
other mice I have tested personally and had little to no acceleration are: Zowie AM, Zowie ec2 Evo, Microsoft intelli 3.0, Razer Deathadder 3.5g-4g, Zowie Fk, logitech g500 and g400.
So it really depends if you're deep into fps gaming. also one more thing. In almost every first person shooter you will notice even at 60 frames there will be mouse delay. I need at least 90 - 120 fps to play effectivly.
Another thing that might help is fixing windows 7 sensitivity (Google it!) and getting a crt monitor or low input lag monitor.