Hi,
The Dell 2405FPW appears to have composite input as well as VGA, so in theory you could use composite video for the Amiga's 15KHz modes, and then VGA for the 31KHz modes...
You'd have to switch the monitor manually between the two, I guess.
Alternatively, you might be lucky and find that the monitor will sync down to 15KHz on the VGA port, since it has to do this for the composite and s-video inputs to work.
Having said that, I have seen old-ish Samsung LCD TVs with VGA ports, and they won't sync down to 15KHz on VGA, so it's no guarantee.
- Ali