Your monitor should have a high refresh rate. Make sure you have a good graphics card too.

Get a high level Sound Card like an Audigy 2 zs or the X-Fi.
http://us.creative.com/products/category.asp?category=1

Speakers - I would go for at least the T5400's.
http://us.creative.com/products/category.asp?category=4&subcategory=25

I would go for a non-wireless keyboard and mouse. If you are really into gaming, get an optical mouse with a high resolution.

Mouse:
http://us.creative.com/products/product.asp?category=10&subcategory=223&product=14489

Keyboard - This is a great gaming board:
http://www.zboard.com/us/