I never found a particularly good answer to this question when I went looking a few months ago.
What I did find, which might be of some use is this article from Microsoft:
http://msdn.microsoft.com/en-us/library/cc966540.aspxThe troubleshooting memory bottlenecks helped me, as did the discussion of DBCC MEMORYSTATUS
The article is geared towards SQL 2000 and 2005 rather than 2008, but I assume that portion hasn't changed much.
(And I look forward to seeing if someone else has a more definitive answer.)
