It partially depends on the OS. Windows Server 2003 was very, very prone to doing massive working set trims for just about no reason, so SQL would just get its entire working set paged out frequently, hence locked pages was near essential (you could leave 50% memory free and SQL would still get tossed into the page file frequently)

Windows Server 2008 and 2008 R2 don't do that as much. They still can, but they're a lot less likely to page SQL out for no reason.

If that server is dedicated to SQL (nothing else running on it), I'd start at 130GB to SQL and see how that goes.