if a were you, i would go the extra mile and get a 6850(i have a gibabyte 6850), it overclocks perfectly, even better than its older brother the 6870, you can check the tom's reviews.
I agree. If you can manage it, step it up to a 6850. I run an XFX 6850 right now, it's a great card that can run basically everything. Not always highest settings, but never lags on medium. Also Newegg has a deal going on that you can get a free copy of Dirt 3 with the XFX- I don't know if it's going on for other cards.
However, if I were to choose from the above though, I'd go with the 6770. The MSI cyclone is great in that it runs very cool, it is probably one of the better 550s, but the Gigabyte 6770 is what I'd go with.
Keep in mind though, that the 6770 is basically a 5770 with some more features, performance-wise, they are very similar, so if you don't currently need all the features of a 6770, you can always save some money and pick up a 5770. Compare the 5770 with the 6770 for features that match your computer's specs and what you need, then, compare the 5770 to the 550ti and see what you like. hope this helps