5.1 surround sound or better - if the game supports this (and a surprising amount of games DON"T!) get it, will double your in gaming buzz. Some have said surround sound headphones work, but I go with traditional speaker set up.
Dual Screens - I know most games don't support this, but having a browser open - even if the second screen is a tiny cheap one, is amazingly convenient.
Some say these make a difference but I haven't tried:
Razer Nostromo
Cyborg M.M.O.7
Cyborg amBX
Logitech G9X Programmable Gaming Mouse - add or remove weights to your preference
Here is where you can see some photo's
http://www.hongkiat.com/blog/gaming-dev ... pc-gamers/
I'm not too fussed by mice really, a mouse is a mouse as far as I'm concerned.
One thing I would love to try is a projector - but I'm being a good saver atm :]