The HD6870 is a good jump from the HD5770 with no issues on 2.1 cards in 2.0 slots. I would swap out the PSU for this XFX 650watt which is plenty big even if you go crossfire with HD6870 later http://www.newegg.com/Product/Product.aspx?Item=N82E16817207002
I ended up going with the setup I posted for the most part, with the 6870 and everything seems to be going well.
Took your advice on that power supply and really happy with it so far. Its a nice unit that comes with alot of extra cords etc. in case I feel like throwing some other stuff in later on. One of the best I have bought, good quality unit and wiring. Would recommend this unit to others.