If you google 550ti then on Nvidia's website it says "Stunning performance for games in resolutions up to 1680x1050"
I guess if you stick to that res you'll be fine, but if you ever get a bigger monitor and go 1080p then anything in Nvidia's x60ti or x70 range is the best.
For the current gen it'll be 660ti or 670. And if you want to go AMD then the 7950 provides a card that's in the middle of both.
why for me would be a problem that i have gtx 560 and x2 250 if i switch to a 1080p screen