It is possible to create a workbook that is larger than Excel can handle. For instance, I successfully imported about 65,000 rows of data into a worksheet where all the data was a single column wide. I then added about 25 columns worth of formulas with no problem. I saved the file, and it occupied about 84 MB on the hard drive.

When I went to reopen the workbook, Excel balked, giving the message "Not Enough Memory," despite the fact that I had previously had the workbook open in Excel when I created it, and despite the fact that my system had more than enough RAM in it.

I was using Excel 2000 at the time that I generated this error. In researching solutions, I found that for versions prior to Excel 2002, there is a 64 MB limit on Excel's memory usage. This limit was increased to 128 MB in Excel 2002, and to a full 1 GB in Excel 2003. Details are found in the following Knowledge Base article:

http://support.microsoft.com/?kbid=313275 

It is interesting to note that these memory limits are per-instance, which means that you may have troubles if you open multiple workbooks whose aggregate memory usage approaches or exceeds the limits noted above. Thus, if you have several very large workbooks, you may want to open them in separate instances of Excel.