Hi!
As Borg said, you would have to write alot of rules to cover the whole forum and it´s URL, but to the rules would be quite simple - here is an example for the rewrite of topic display:
RewriteRule ^forum/forum_topics-FID-(\d+)-PN-(\d+)\.html$ forum_topics.asp?FID=$1&PN=$2 [NC,L]
The default URL would be: ".../forum/forum-topics.asp?FID=20&PN=1
With URl rewrite the URL would be: ../forum/forum_topisc-FID-20-PN-1.html
Although this is simple, you also have to edit the URL´s in the source-code of the forum, as they don´t get rewritten by default, which is quite some work!!!
If the purpose is to gain better ranking on searchengins, I surgest to take it even further, and include the name of the topics in the URL!