Oh and since it wasn't mentioned, don't get the naga mouse if you are looking to upgrade.
It is an amazing mouse in theory, but it doesn't last. Four of my friends, hubby, and myself all got one around the same time. Mine was the last to survive after about a year and a half.
Hubby and I switched to the g600. I still like the naga's layout better, but the g600 seems more solid.