When do my directories become link farms?

[directoryczar]

I work for a couple of niche directories, and one in particular needs a little help in the serps. I have access to a lot of highly relevant articles that I would like to use to draw buyers and bots to my directories. The home pages of both websites are, happily, full of premium advertising and so I don't have room to try and add content to those pages.

I'm talking about adding 5-10 pages with less than 100 links each, the domains in question are PR 6 and PR 4 with hundreds of outbound links each.

Would I be better off

a) creating subpages on the domains to feature the content with lots of static, optimized links to categories and pages within the two domains?

if a) does heavy reciprocal linking between the two domains create problems for me?

b) creating new domains and putting the content and links there?

if b) do I need to get a separate web host to get the most effectiveness out of this strategy?

[wuzexin2000]

past 100 links

[thedagda]

past 100 links

Is this a hard and fast rule? Where did you find this information and do all SE's use the same number?