Super Duper Uber Mega Ultra Extreme
There are two things that matter when you're picking out a gaming mouse:
The first is the DPI and how far the DPI does & doesn't extends. If you're buying a gaming mouse and the DPI only extends to 1200 than you might as well buy a regular mouse because that slight increase isn't, however, you don't want to set the DPI higher than you're comfortable with. For example I have played with a mouse that had a DPI of 5700 with low sensitivity* and I just couldn't get the hang of it, but @ 1920x1080 I feel comfortable with a DPI of 3200 with moderate sensitivity. If you train yourself to react to a higher DPI & high sensitivity it will, technically, mean that you can perform the same movement faster with less force.
The second, and IMO, the most important thing about picking a gaming mouse is the weight system. If you're buying gaming mice just for the DPI than by-all-means go right ahead and plop down that wad of money. I would rather buy a 2000 DPI mouse with a weight system than a mouse with 6000 DPI without one. The weight system can be compared to IRL sensitivity settings. Normally it will come with five to eight weights that can be removed or entered into the back end of the mouse. This allows you to adjust the push and pull weight to your liking. My mouse has eight weights and I use four of them when playing FPS games and six of them when doing regular tasks.
A common misconception about gaming peripherals is that you have to spend a boat-load of cash. Razer makes great peripherals but they cost a lot. You can easily find cheaper mice and keyboards. I would personally suggest the AnkerŪ 8200 DPI mouse because it has an eight-piece weight system and that super high DPI can be adjusted to your liking. It's not going to win you mad cred at lan parties but it's functional. Mechanical keyboards can be had for as little as $30 (maybe lower?) on ebay, but that's another thread.
*Sensitivity is a windows setting and not a mouse hardware setting. Sensitivity allows you to control the the accuracy of the mouse, while DPI is how fast the mouse moves. Here is a quote from someone who can explain it a little better than I can: "DPI means dots per inch. If the mouse is calculating more dots per inch then it is more precise. A bump in sensitivity is just a change in how the mouse interprets the jump between dots. So, low sensitivity with High DPI wont be any faster or slower, but it will technically be picking up more precise tracking."
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I have the Mionix Naos 5000. It glides over my mouse pad much smoother than my old mouse. It feels really comfortable as well. If you're playing FPS, a mouse that you're comfortable helps tremendously. For MMORPG or RTS where you want to have a lot of key bindings, mouses with extra buttons are very nice.
Gaming mouses with a lot of features are more for hardcore gamers. A regular mouse will be fine for a normal gamer. If you're not very hardcore but use the computer for a long time, a comfortable mouse is very preferred.