I have 27" (Main) and 21" (Second)
It depend so much, what you want to do with that?
Just second monitor like I have (Pic), or do you want to make it WIDE?
Sry about bad pic quality :)
I got my second ages ago, from work... Not good, but hey... it works!! :D
In pic left, u see 3rd monitor, Old School 21" (Deeper than wider) and omfg its Heavy!!
I can Email that to you if u need!! :DDDDD
But if you need Sharp/Good Picture... Go for old school... :D