garrzilla :
What card do you recommend then paring with my 550 ti? Another 550ti? I would like to be able to use all 3 monitors in fullscreen games
For just the Windows desktop, any card would do, e.g. GeForce GT 610.
For games to run on the three screens though, you need to enable Nvidia surround.
For this you need an SLI setup so the two cards need to be identical.
Trying to run games on three monitors with such lowered cards though, you are going to be severely limited with resolutions and detail settings.
Far Cry 3 at 2560x1080 resolution (3 screens at 1920x1080) with ultra settings
Two GTX 660 cards in SLI can manage and average of 15.1 frame per second.
http://www.tomshardware.com/reviews/far-cry-3-performance-benchmark,3379-6.html
Newer cards like the GTX 700 series support up to four monitors on the one card.
These only have two DVI connectors, but you could use HDMI for the third monitor.
With two of these newer cards in SLI, you could use two DVI connections on one card and one DVI connection on the other card.
Benchmarks of what you can expect:
http://www.guru3d.com/articles_pages/geforce_gtx_770_sli_review,22.html
To give you some reference point to your existing card, a GTX 770 is roughly 3 times the performance of your current card.