As always it depends on the ‘quality’ of the keyword. So for instance ‘100% casino bonus’ with 100 searches/month could be or probably is better than ‘casino’ with 200 searches/month.
Personally, use http://www.digitalpoint.com/tools/suggestion/. After I put the serach term in I multiply 6 by the number of searches/day from the wordtracker section (left column) to get a rough estimate of the monthly searches in Google.
So in your case, 1200 visits/month is 40/day. So that equates to roughly 240 visitors/month.