I was a real addict of forum posting and have more then 2500 to 3000 posts on most of the forums. If the number of posts is high enough in your forum posting and on more number of forums then it might be useful.
Calculations:
3000 posts x 4 links in Sig = 12,000 links.
12,000 links x 10 forums = 1,20,000 links.
Each thread on an average viewed by 200 times.
1,20,000 x 200 = 24000000 impressions for your links.
Lets assume a Click through ratio of 1 to 2 percent.
For 1% CTR it will be 2,40,000 visits.
For 2% CTR it will be 4,80,000 visits.
Please let me know your ideas on the above assumptions. It is just a speculation. But that are the figures that I have seen and have experienced so far.