Personally, I don't think there's any point in upgrading, just to upgrade.
The real questions are: what cpu you do you have (so you can determine if upgrading your GPU is even going to do anything for you), and are you happy with your video game performance today?
That said, a 550 ti is, to put it nicely, a piece of junk.
Or, at least, not very good, even in today's weird PC video game world where games are often built with consoles in mind and thus not really requiring the higher end tech to run well on a PC like the good 'ol days.
Which makes the first question, concerning your cpu, even more relevant. If your cpu and video card weren't noticeably bottlenecking each other, just upgrading your video card can have only negligible effects, especially in the case of a 550 ti.
As an aside, these days, I'd consider a GTX 560-$122 on newegg-(not the GTX 560 ti), the budget video card of choice. And, it's significantly better than a 550-ti plus.