I beleive also that one IP for all domains also hurts SE rating..
not sure why .. cant remember actually but I think you are better of on seperate IPs for each domain
I have acheieved a #9 rating on google with domains on the same IP ..
but rank really seemed to bounce up and down.
Check out this article ... I think its outdated.. but I got #9 google after reading it a few times and tweaking some pages.
http://www.apromotionguide.com/
not adult oriented , but either way usefull .