Infrared/optical mice tend to have lower DPI than laser mice, but also provide much smoother movement and more precise movements. A higher DPI setting means you can move your mouse a shorter distance while still moving the cursor the same distance on the screen. Simply and broadly put, if you move your mouse 5 inches to get your cursor from one end of the screen to the other at 500 DPI, at 1000 DPI you only have to move your mouse 2.5 inches to do the same thing. The downside in this is a loss in precision and/or smoothness, depending on the mouse.
Lift off distance is the distance it takes until the optical/laser sensor stops registering movement. A low distance means you have to lift your mouse less when repositioning the mouse on your mousepad.
Acceleration lets you retain finer movements while moving the mouse slow, but the faster you move the mouse the pointer will ramp up its movement speed. Most gamers prefer to leave acceleration off as it can kick in unpredictably in games, and doesn't always provide the expected smooth acceleration in mouse speed. Your mileage may vary, and it's preference dependent.
1. Type of grip (palm or claw). Do you rest your whole hand on the mouse, or do you "claw" the mouse, moving it mostly with your fingertips?
2. How many buttons would you like?
3. Do you like your mouse heavy or light (most palmers prefer heavy, while claws usually go light).
4. What is your sensitivity setting in Windows? Or, do you prefer to move the mouse a little to move the cursor X distance, or do you prefer to move the mouse a lot to move the cursor the same distance?
5. Do you lift the mouse off the mousepad to reposition it? If so, do you like/dislike this?
6. Budget?
7. Extra gizmos (rubberized surface, adjustable tactile force, etc).
8. Gaming on other's systems where you can't install drivers?
8. Wired/wireless? You'll get better results probably with wired, especially if you go high DPI (> 2000).
Depending on your answers, you can go several routes. The Logitech MX518 and now the EC1/2 from Zowie are very good infrared/optical palm-style mice and operate without drivers. Both can change the DPI setting on the fly without drivers, and both run around the same $35-$40 price point while the EC1 and 2 are on sale at newegg. The EC series steps from 500/1000/2000 DPI while the MX518 caps out at 1800 DPI.
Infrared/optical claw grips would probably be suited with an Abyssus from Razer, or maybe a DiamondBack (if I got my line right). Both use the 3.5 generation sensor and have smooth tracking.
Laser mouse wise, you have more options and all are high DPI. There's the Steelseries Xai which is very popular with claw gamers due to light weight. From Logitech, there's the G9x and the G500. The G9x tends to be much more customizable and leans a bit to a claw grip and has adjustable weight, while the 500 leans to a palm grip. Razer just came out with the Spectre and Tron mice, both of which are lightweight lasers. The Imperator is one of Razer's preferred claw style laser mice, and the Mamba is a favored wired/wireless palmer.
There's also the Naos 3200 and 5000 which I've seen some praise on. The 3200 caps at 3200 dpi (laser). The 5000 caps at 5000 DPI, has adjustable weights, and also includes some additional software configuration and profile storage.
If you do gaming on the road with a laptop, the Orochi is hard to beat. Wired or wireless, up to 2000 DPI when connected via bluetooth, and not *too* heavy with batteries. Definite claw style mouse though. It'll probably hurt your hand if you use your entire palm.
Since most FPS games nowadays are fairly CQB-styled, you'd probably want a lighter weight mouse with on-the-fly sensitivity/DPI adjustment. Also, you have smaller hands, so I'd say the EC2 (palm or claw)/Abyssus (claw) if you want optical, or the G9x (claw or palm)/G500 (palm)/Imperator(palm), or maybe one of the newer Razer laser mice (I haven't tired them or read up on them yet). I tend to prefer the Logitech drivers over the Razer ones, and Logitechs have tended to fare better over time than all my Razers.
*Personally*, I tend to be a hybrid claw/palm twicher, though I find 2000 DPI the cap at what I'd like to run at (depending on the mouse). I was using an Orochi, but got in an EC2 last night. I'm finding myself to prefer the optical EC2 as the grip fits smaller hands, lets me do my hybrid grip, and has an excellently smooth track and tactile button feel. Running the same DPI as the Orochi, I find it tracks better and feels much smoother in cursor movements, though the "feeling" bit could just be placebo.
The thing is, a mouse is quite a personal preference, perhaps moreso than a keyboard. Your best bet is to try to find a store that has a selection in stock that can give you a good grip test to see what's most comfortable in your hand. If ordering online, you might want to try to order from a website with a good return policy lest you wind up with restocking fees or a collection of unused mice (like me ). Many top competitive gamers still use Intellimouse Explorers (1.1 and 3.0), and those are really, really basic mice.