should I put my mem at 300mhz or 400mhz I have seen both used on 6870's
It's half looking at what other people have done and half experimentation to find values that actually work well for your card.
Use the other examples you've seen, try each, and find the numbers that give you the best hashrates. Every card is different - some overclock well, others not at all, etc, you need to experiment.
That said, lowering your memory speeds reduces power consumption, so try to find the lowest value that's stable on your card and doesn't degrade your hashrates.