Personally not a fan of SLI/CF, I've had major problems in the past (especially with CF). It may have improved in the last two years since I used it but I avoid it like the plague. Just wait until the R9's come and buy the best one of those you can afford. It may not quite be up to the same performance level of two 6870's but you know what? You will never have to deal with the BS compatibility/driver/waiting for CF profiles etc. Heck the new R9's might actually out perform two 6870's for all we know.
Or just buy a 7970/7950 right now since all the AMD cards are under 300 right now (at least in Canada).