Well, for me its all about the pixel count.
With these 21:9 monitors the vertical pixel count is stil 1080, like any full HD monitor.
Only the horizontal pixel count with 2560 pixels is 640 more then 1920 with full HD.
Thats only 1/3 wider then a comman full HD monitor.
As an eyfinity gamer with 3 full HD monitors the 21:9 screen is not interesting.
But if cashflow is limited... not only for the monitor but also the VGA card.. i understand this could be an option.
Most games will probably support 2560*1080, as they also do 3xHD (landscape or portrait).
But if you have the cash i would go for an 27 inch 2560x1440.
Same horizontal resolution but 360 more pixels verticaly!
(or 3 full HD screens
)