Noone can tell except google. There factors such as traffic, ctr, nature of site, nature of traffic, content, etc. Just by looking a site nobody can tell you howmuch you can earn in a month.
Commonly! forums doesn't give much even on Adsense! i tested much times but it gives low just only for CPM's
Impossible to say but often people are visting a forum for information and not to purchase so they ignore the ads a lot.
yup forums don't earn much.. id suggest you to start a blog. which is more profitable in various ways. Newcity,
forums have the worst ctr. usually 3% or lower. regardless, try it and see for yourself. who knows you can make good money from adsense.
If you could put some ads in the right places maybe you could gain some bucks. What are your monthly traffic?
I'd suggest getting with an advertising network who pay for 1,000 views, that might bring in more money.
Adding link units under the first post will generate revenue. Guessing the revenue is a wild guess without knowing moe info about the traffic.