I am running 3 2405s off a 5870. My system specs are below:
Q6600 @ 3.2
4gb ram
5870 @ 875/1200
You will need a DP-->DVI adapter for one of the monitors. If the DP-->DVI monitor has blinking issues, you will need to flash the bios to fix this problem. This problem/solution has been discussed thoroughly in the forums.
I play MW2 (at max setting) almost exclusively and generally performance has been good. There are some areas of slowdown but I think it has more to do with my underpowered CPU as opposed to the the 5870.
Depending on your cpu and they type of games you play, the 5850 may or may not be enough. If you like to play will all the eyecandy maxed, it most likely won't have enough juice for smooth framerates.
You are right, performance at 5760x1200 and 3600x1920 will be the same.
I have my third 2405FWP coming soon. Can you tell me the brand of DP-DVI adapter you are using?
Thanks!