hmm, I dunno about that.. I still play Counterstrike 1.6 every once in a while.. and it seems like all the most accurate, sharp shooting guys that drop you with a single shot to the head all insist you need high, but not unnecessarily high dpi.
So I would say it only matters to a certain extent. The inordinately high DPI count is just a marketing ploy imo, to appeal to people who blindly accept statistics to be performance indicators.
Anyway, my favorite mouse is the MX518! But also because I'm just so used to it.. Can't believe they took it off the market!
So I would agree.. go with whatever button config you like, but even more importantly: what feels best in your hand.. and you'll kinda just grow into it like I did with my 518 and not want to use anything else afterward.
You're right about the dpi comment and there was a writeup on it.
http://www.esreality.com/post/1265679/esreality-mousescore-2007/?a=longpost&id=1265679&page=2
There was also a poll years back on what the best fps players used and many used relatively cheap mice ~800dpi - low sens + large movement allowed them to achieve high accuracy for headshots.
I like the mx518, but I prefer a mouse that has tilting scroll wheel buttons, no-angle snapping (which the mx518 has, with no way of switching it off) and an option to configure thumb-hand pos like the Rat7.