Hello I'm thinking about buying another graphic card 640 gts with 1gb ram, but keeping my 550 gt with 2gb ram. I'm using two 24" and one 32" inches monitors. I'm going cheap on graphics cards my budget is about 259,93$ I'm a Newbie!
U would have to buy new gpu since u cannot SLI the 640 and the 550.
However,the truth is within 260$ u cannot get two powerful gpu or a single gpu on which u can game on two monitors. What u can do is u can get something like the hd7950Boost/GTx760 and game on a single 24" or aon a single 32" monitor. See that if u can find any deals on the hd7970/gtx670.
Two gtx680's is powerful than a single 780. So,if u can afford it,then go for it.
But do SLI using two gtx680's only if you find it somewhat cheaper than the 770, otherwise do SLI with two gtx770's.