Ram is ram IMO. If it has a lifetime warrenty. . . then there isn't much difference from one brand to another.
Some have specialized IC's which can improve overclocking. . . but considering the actual memory chips generally all come from the same company/place, it doesn't much matter (at least to me).
I've gotten tons of DOA ram from all companies. Mainly PQI and SuperTalent though (and a few OCZ sticks have been DOA I suppose). Will I stop buying from any of these companies? No, not really (except for supertalent because one time I had faulty ram which tested okay in memtest . . . though that doesn't matter all that much). Once you get working ram. . . it just works - unless you get a power surge that fries it. . . eh
I've bought ram from all these comapnies (sorted by high volume first)
OCZ
PQI
Corsair
G-Skill
Crucial (HP OEM)
Kingston
SuperTalent
A-DATA
If you feel the need to go for 6GB, then go for it. You can generally missmatch ram from different vendors with different timings and different voltage, but I like to think of the best practice as getting close timings with exactly the same voltage (and just running the ram at the slower timings obviously).
Running 8GB rather than 6GB doesn't really have any advantage other than the obvious extra 2GB. So like I said, if you want it. . .go for it. I've ran some rather strange memory configs such as 2x512MB+2x256MB (1.5gigs), 3GB, 6GB.
Ram is generally the last thing I care about. It doesn't matter much for overclocking or even for speed. The only programs that get an increase in performance with higher speed ram are synthetic benchmarks.