Oddly I just upgraded from a 5770 and acquired two 560TI's (EVGA, 448 core ones) for $100 after shipping from a friend. Only using one however, since I don't have a second PCI-E slot.
Personally I know damn well I'm CPU bottlenecked, but I'm still noticing *some* performance improvements in games. Plus, and I don't want to start anything with this, but at least being able to get updated drivers is a plus.
The newest drivers for the 5770 were from like, March of 2013, and as such BF4 b****ed at me every time I played it because my drivers were 8 months older than the game.
Ultimately, don't expect a whole hell of a lot and it may not really be worth it for that card at that price. You'd almost be buying it just to be switching to Nvidia and/or getting something slightly newer that still has some limited driver support, if that's anything to you.