wait what!
21.5" is a 1680x1050 monitor, no?
A single 5770 would be more than enough for most situations, and a single 5850 would probably have you maxing even Crysis w/ all the extra eyecandy.
Yeah, that's mega overkill.
if you were on a 2560x1600 30" I'd recommend a 5970 or 5870 CF, but not 1680x1050.
A single 5870 will be much more than overkill. You'll probably be able to apply heavy SSAA to even new titles w/ that much overkill.