I have the MX900 and it works very well (comes with a bluetooth base as well). The MX700 has gotten great reviews as well. The corded MX510
was well liked as a gamer's mouse.
I am also an FPS player. Even though it's wireless it's nice for the twitchy games.
I had the latest MS mouse
and it was mushy and slow and I died to easily in UT2004 so I ditched that. Scrolling for a new weapon most often lead to me trying to kill the guy with a rocket launcher by bludgening him with my fists... frustrating.
But, if you are already using MX500 your best and possibly cheapest way to go would be to try to support your arm/hand more or move it to a more neutral position. Ergonomics might be poopooed by some people, but it can make huge difference. (hehehe I wrote poo)
Or do what I did. Suffer through the pain until you do so much damage that you are numb.....