I'm curious whether enabling search engine friendly URLs will overly tax the shared server I'm on. I've been leery to enable it, but think it'd be important to get better SE coverage. Also - will my current SE links still work?

existing links will continue to work & no, I don't think it will affect your server.

It won't really effect your server; it just changes how url's are published from threads.

Using a tool such as a sitemap will also assist in getting links published into SE's (:cough:)

assuming you have the latest version

If you have SE friendly URLs, won't that mean more spiders will crawl your site, hence increasing the load?

Well, they'll hit your site reguardless, you simply help them with direction to content ...