Welcome to the forums.
First of all, you need to figure out (a best guess) as to how you'll hold the mouse when you're gaming. There are generally three different ways you can hold a mouse, and each has different mice specialized for that position.
Read through this article regarding mouse position (palm, grip, fingertip), and let us know how you hold the mouse (or a best guess). The most common style is the palm grip, which the Razer Deathadder, Logitech G500, and Logitech MX518 are best for. Note you can use more than one grip style on the same mouse.
I for example tend to use a combination of a palm grip and a claw grip. I have a Razer Deathadder, and it's good. I just bought a Logitech G500 though, and it's hands down the most comfortable mouse I've ever used.Edited by Xyxyll - 2/23/11 at 10:27pm