550 TIs will be just as outdated. AMD is just ahead with their 7000 release. Nvidia Kepler will be here soon. they are both last generation cards. you could have waited for a 7850 but it would be 250 probably. would also have better performance. so really. no. you could gave waited for the new cards, but a 550ti is no better
Today tom's published a review about the mid range 7000 series, namely 7850-7870, and it seems that, at least the 7850 is dissapointing (hanging up and performance issues), also they're not going to be available until the 3er week this month. As unksol stated, nvidia's kepler will be here by may possibly competing with the 7000 series, so i wouldn't be buying any of those until the price fight between them stabilizes, which i suppose is going to happen by the end of this year.
So, if you've purchased a 6870, i think is a good buy now and you can upgrade to 7000 series (or kepler if you like) by next year (also prices and drivers will be better by then)