955 with 570 defo!
The 570 beats the 560 TI and 955 will not hold it back in any way shape or form so you'll have a better gaming experience as you'll be able to top out more games.
Not sure if this has already been covered as I just skipped to the end of the thread to get my 2 cents in but if you want to be able to add a second 570 later then you'll need a 990X board as the 8XX mobo's don't support SLI. Also, a mid range 990X board will run two cards at x16/x16 where a mid range P67/Z68 board will only be running them at x8/x8. Doesn't really make a difference over one monitor however but still, there you go. Sorry if you already knew all that but there's never any harm in checking is there?