You should be fine to add a second 6870 with an 800w PSU. Granted it's not the best 800w unit available, but it should be fine.
I am also considering overclocking the 2600k (to around 4 Ghz) alongside the new GPU, would this sway your opinion any on purchasing a better PSU?
Also, I'd be very grateful if you could list a couple of PSU manufacturers and product lines that are better recommended. Educate me! :]