Network Card Ordering

[Dave]

Hi all,
 I am using FreeBSD 7 and all my computers have built in intel lan
cards. When you boot you get em0 and em1. On some pc's I add an
additional network card which shows up as em2 and em3.

The problem I have is that on certain hardware that network card will
take the spaces em0 and em1, and the onboard cards will shuffle. Now
for various reasons this is a problem - is there any way to
specify/force cards on boot. I don't want to rename them because I
can't tell when this will/wont happen; I would rather force it somehow
to prefer the onboard cards?

Thanks for the help
Dave