You don't have to break the bank for a 1680x LCD either. I prefer the 20" screens due to the better pixel pitch. Here are a couple that would be good for gaming-
Acer for 190usd
BenQ for 170usd
Both of those models claim 16.7million colors too. So the colors should be good for what they are. I'd recommend using the DVI connection.
An 8800GT @ 660-700|2000 1650-1850 shader should be plenty of card for 1680x in most cases. Crysis all very high being the exception.
I was able to run AoC @ 1920x will all on and all enabled with the rest set to "high" @ 1920x on an 8800GT @ 700|2000 1750 provided I kept off AA. It did fine with 8-16xAF.
If you decide you must get a new video card for whatever the reason, try the below HD 4850-
It runs cool and should do over 700 on the core when overclocked. It's 180usd with rebate.
Another great HD 4850 for the cash-
It's 163usd with rebate.
Below you can see the HD 4850 compared to a stock 8800GT-
Review of the Sapphire Toxic HD 4850 is below-
Most of the time it will give you superfluous FPS @ 1680x. If 8xAA is your thing, you might want to consider it.