You could sell your current card for a reasonable sum, and put it towards a better single card solution that will fit your needs better. 550tis aren't exactly the most powerful cards. 2 550tis could potentially bottleneck the power of your i7. If you want to run multi monitor at high settings with your 550ti's you may not have a good time.
Still needs two 500 series cards to run three monitors. 600 series can do multiple monitors. Sorry, cat, I don't have a lot of expertise with multi-monitor setups.