TBH, going to 5 forums and getting involved more with the community is better than going to 40 forums and just posting for the sake of posting.

Generally - People visit sig. links from posters that have either helped them directly (answering questions) - Are prominent and prolific posters on the forum, or the sig is really eye catching. - but if everyone on the forum has flashy banners and 'attractive' links in their sig, people generally ignore all of them.

It depends on what you're trying to "pimp" i suppose. That's from a "human" point of view.

As for search engines - i'd be inclined to think that having links on 40 sites is better? I can't say i've experimented with stuff like that.