The one you have now seems to be a pretty decent monitor.
For the dual monitor setup what are the main purposes going to be?
If photo/video editing i might look at something with a IPS/SIPS panel rather then a TN one.
If everyday stuff, gaming and browsing i would be tempted to just get the same one.
Unfortunately i have been out of the market for monitors for a while and cant give any recommendations on the IPS but usually samsung, dell and lg are the ones i would be looking at for those.