In your case, as previously suggested by many, the simplest solution may be to add more RAM and extend the life of your spreadsheets, regardless of size.
How much is enough? I opened Activity Monitor and watched the little pie chart. At no time during the operation was all of the RAM used. It nearly always showed at least a combined 30% Inactive (blue wedge) and Free (green wedge), yet I felt that something happened to the computer more extensive than an application crashing. The Finder acted like it had problems, too, so I restarted and everything went back to normal. How much IS enough?
