Yes, yes, yes, yes, and yes.
A good mouse and good keyboard DEFINATELY contribute to your computer experience. You can have the nicest display, the greatest speakers, and the fastest internals, and have them all basically go to waste if your keyboard and mouse don't perform. These are the #1 and #2 ways of interacting with your computer.
With that being said, try an old, generic PS2 2-button beige ball mouse. Use it for a week. When the week is up, switch to something like a Logitech G5, Logitech G7, Logitech MX518, etc. Try those for a week, and decide which you like better.
While mice are much more performance oriented, I find that keyboards are more comfort oriented. Some of the more comfortable keyboards I've used are from the recent Microsoft Laser (Wireless) sets. They're not my favorite, but they're definately easy on the hands and wrists. I personally use a Logitech G15, and I love it. It's not as confortable as the Microsoft options, but it gives me better gaming flexibility with the customizable keys, and the LCD (which does come in handy, especially for Ventrilo).