I remember reading this somewhere but I forget the "magic" number. How many pages does it get to be when you have to break your sitemap into two files? Also for me, I use a XML sitemap generator (www.xml-sitemaps dot com) - what do you guys use? A generator or do you write it out?
We use gSiteCrawler for all of our clients without problems. It's free, no limit and handles multiple sitemaps automatically. Think it is 10K?
It's 50,000 per file at Google. http://google.com/support/webmasters/bin/answer.py?hl=en&answer=34658