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Don't go for the cheapest mouse you can find. It's useful to have more than the regular number of buttons on it so that you can put the most common/natural actions on the mouse.
Me, I've been using trackballs for work and gaming for years. This kind:
I find trackballs much more accurate than mice. Also, since it doesn't move around the table, for gaming I can keep the trackball close to the keyboard, have my left hand over the left of the keyboard and my right thumb (less often my right forefinger) handy for a quick stab at the keys down the right side of the kb.
How have people found going from controller to Keyboard and mouse? I mainly play FPS and looking to swap over, got my eye on a cheaper keypad & mouse combo to start with and see how I get on.