Two things.
1.) Unless you're running SoftTH, you do NOT want monitors with different resolutions, let alone different aspect ratios. And SoftTH doesn't work with all of the games TH2Go does.
2.) I would suggest not getting monitors that aren't identical to the one you have, as the height discrepancy will irritate you (disregard this if you're planning on buying a multi-monitor stand).
Monitors are also (relatively) inexpensive if all you're doing is gaming. Have you seen Dell's spring outlet sale
here?