Your monitor's max resolution should be 1920x1200.
Couple of things- depending on the games you are playing, 1 relatively high powered graphics card should be enough. Something in the $150-250 range. However, if you are playing some of the most graphically intensive games with all settings turned up, you may start to reach the limits of what that caliber of card can do. Identifying your resolution and the types of games that you play is a good way to start narrowing down what level of performance you need (and therefore what kind of graphics setup will suit those needs).
I'm not sure why you are asking us. Does your current setup give performance you find acceptable? If not then consider a second card. At that resolution it's really a personal judgement call only you can make.
Well, it really depends on what settings you are willing to live with. A single GTX 560 Ti is quite capable for 1920x1200 but in the most graphic intensive current games you will likely have to set AA low or off. A small handful of games like Crysis(the first one) and Metro 2033, AvsP you may have to turn down some other settings as well. Basically if you are the sort that needs to max out every setting in every game then just get the second card. If not then try the games you are interested in with the single card first and then decide.
not to hijack this question but would the Nvidia GTS 450 be able to run at the 1920x1200 resolution (same monitor) easily? I am getting the time out errors and am wondering if in fact, it's the card that is unable to perform at those ranges. I'm not even playing games.