Optical VS Laser- Optical mice are generally going to be more accurate and better for gaming than laser mice.
What Dpi should I use?- Most professional players prefer to use low Dpi settings as opposed to high. Dpi stands for 'dots per inch', which basically is the sensitivity. It's a measure of the distance your mouse moves in relation to the distance your mouse moves on the screen. A good range would be between 400-1200, depending on the game.
What's Acceleration? Do I need it?- Acceleration takes mouse sensitivity a step further. The faster you move your mouse, no matter the distance, the further the cursor will move. An example of this would be if you move your mouse 2 inches in 1 second, the cursor would move 500 pixels. If you move your mouse 2 inches in .5 second, the cursor would move about 750 pixels. Acceleration usually leads to a substantial hit in accuracy, and it's advised to disable it.
What's Interpolated Dpi?- Interpolated Dpi is created by the use of software to increase the Dpi of the mouse. This greatly decreases accuracy of mice and you should make sure you have it turned off.

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Why a trackball? Why not a mouse?- You don't necessarily need to switch to using only a trackball. It's a nice alternative to use instead of your mouse for browsing and normal computer tasks. Sure, a trackball will never beat a mouse for gaming, but they're so much fun to use, you'll never regret the purchase.
What are the advantages over a mouse?- Trackballs stay in place. You don't need to move them around, so they're very effective in areas where you don't have much room. If you suffer from carpal tunnel, trackballs are a great alternative to mice because it relies less on your wrist and more on your thumb or other fingers. No wrist movement == no wrist pain. There are many other advantages as well, which I won't go into detail about.
Should I be a thumb baller or a finger baller?- It's your decision. I myself own the M570, a thumb trackball, and love it. Now that I own a thumb baller, I really want to own a finger trackball as well. If you're not sure which to get, go with a thumb trackball first and see if you would like a finger trackball better.
What are thumb trackballs and what are finger trackballs?- Thumb ballers are trackballs such as the [ Register or Signin to view external links. ] , while finger ballers are trackballs such as the [ Register or Signin to view external links. ] .

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What is a mechanical keyboard? Should I get mechanical or rubber dome?- A mechanical keyboard is a keyboard that employs mechanical switches underneath each key, as opposed to rubber membranes. Most keyboards aren't mechanical. Under no circumstances should you ever spend more than $15 on a rubber dome keyboard, EVER. Everybody should own a mechanical keyboard. The typing experience is greatly improved, as is gaming.
Which mechanical switches are which?- Refer [ Register or Signin to view external links. ]
Rubber Dome Keyboard vs Mechanical Keyboard- [ Register or Signin to view external links. ]
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There are many different opinions about every pair of cans. Before you purchase anything, remember to look up multiple reviews of the product from various sources. Talk to somebody who owns them and see what their gripes are with it. There is nothing subjective about headphones, and I don't see it happening any time soon.

r00t wrote Nobody measures things and so many things can't be measured.

Until a means of measuring accuracy of headphones and understanding their output of audio, you're gonna have to go off of whose opinion you like best, and seems to be the most knowledgeable. With that in mind, here are some links to some gaming and music headphone guides. Remember: take everything with a grain of salt.

[ Register or Signin to view external links. ] - I would also like to note that this should be the only thing you're reading from head-fi. Their community has taken a dump and is not worth reading anymore.
r00t's Advice Thread
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I don't know enough about the current headphone market to be recommending anything right now. Check the helpful information spoiler for a few good sources to choose from.