There are a few reasons why a 2405 needs alot of graphics power
#1. 1920 by 1200 is ALOT of pixels. In comparison, its around 20% more pixels than 1600 by 1200.
#2. Alot of games aren't optimized for widescreen. If a game is able to support widescreen, there -might- be a small performance hit for going widescreen. Would be nice to see a quality article on this...
#3. LCDs look best at resolutions such that a single pixel on the screen can be matched with a single output area. In the 2405's case, that means 1920 by 1200 or 910 by 600 (2 display pixels for each signal pixel). Other resolutions won't look as good. Since your probably not going to want to play in 910 by 600 (if the game can even support this) then your "stuck" with the higher resolution.
Some points in favour of the 2405.
#1. You can do 1:1 pixel scaling. This means that you can do different resolutions, but with black filler around the edges. Note, this is LCD black, not off, so some light leakage from these sections may happen.
#2. It has alot of connections
#3. It has been widely reported to be a "better" resolution scalar. This means that non-native resolutions are generally better than other monitors. Its a personal tolerance issue mainly.
As to the 2001FP, it has a pretty poor viewing angles and black levels that are for me unacceptable. Otherwise, it is a servicable monitor and depending on your level of sensitivity to black levels could be a good choice.