You can't crossfire 5770 and 5870.The cards have to be the same.(You can find more info about SLI/CF in my sig)
However,you can get another 5770.(2 5770s will give you the performance of 1 5870)
You think that will be good enough to play just about any game out there for now w/Phenom II 965 and 8 gigs ram?
My mobo has an ATI 4850 onboard which I assume does not even work with the Safire plugged in. BTW, nice explination on SLI/Crossfire you wrote.