The 5770 is an energy efficient card that packs quite a punch - it's about the same speed as the 4870 that I have in this rig and to be honest I can still play almost every game at very high settings with an acceptable (to me) framerate, but then I don't really play shooters.
I know less about the 550ti, but I think it performs at least as well, probably faster in some titles, at the expense of more power being used, which leads to more heat and maybe noise. But that might not be a concern.
See the hexus review here:
http://www.hexus.net/content/item.php?item=29509