not an expert but happen to know the answers:)

1. stock A, bought 600 shares in August, 2008. Sold 400 in Oct., loss -$1000. Still have 200 shares in account. Can I get $1000 as Capital losses?

yes

2. Stock B. Bought 200 shares in Oct.2008. The price keeps going down, then bought 150 shares to lower down the average price in Nov. I sold all of them today, and total loss is about $800 (200 shares loss $1500 and 150 shares earned $700, and Please notice that I held the 150 shares for 27 days).
Can I claim the tatal loss of $800 as captial losses?

yes

3. If I want to re-buy Stock B, How long should I wait to get away from "wash sale" ? 30 days after selling?

yes, on day 31

4. Can I use the money that I got from selling stock B to buy other stocks recently?

yes

the wash sale rule basically prevents one from owning the stocks AND taking the loss at the same time. if you want to take loss, then you have to let go of the stock. can't have both worlds.