Advanced: SEO best practice for a large forum to minimise risk...?

Advanced: SEO best practice for a large forum to minimise risk...?

Hi

Hope someone can offer some insight here. We have a site with an active forum. The transactional side of the site is about 300 pages totals, and the forum is well over 100,000 (and growing daily) meaning the 'important' pages account for less that 0.5% of all pages on the site.

Rankings are pretty good and we're ticking lots of boxes with the main site, with good natural links, logical architecture, appropriate keyword targeting.

I'm worried about the following:

* crawl budget
* PR flow
* Panda

We actively moderate the forum for spam and generally the content is good (for a forum anyway), so I'm just looking for any best practice tips for minimising risk. I've contemplated moving the forum to a subdomain so there's that separation, or even noindexing the forum completely, although it does pull in traffic.

Has anyone been in a similar situation?

Thanks!