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/
I have my PC. However I need a monitor, keyboard, mouse and speakers.
All of these need to be suitable for intense gaming, Internet (Email, web, instant messaging), and Productivity (Word processing, spreadsheets, etc.

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