Refer to http://www.decisionmodels.com/memlimitsc.htm

"Excel 2003 is officially limited to 1 Gigabyte (GB) of memory. This limit
appears to a limit on the working set memory used by bthe Excel process,
which is the memory reported by Windows Task Manager.

Excel 2007 is limited to 2 Gigabytes of memory for the Excel process under
Windows XP (Windows memory limit). This 2 Gigabyte limit is a limit on the
Virtual Memory address space. Virtual memory used by a process is larger than
the working set memory reported by Windows Task Manager, so the amount of
useable memory under Excel 2007 is considerably less than twice that of Excel
2003.
Because Excel 12 also requires more memory to store the indexes to the
increased number of rows and columns you may not be able to load larger
workbooks under Excel 2007 than was possible under Excel 2003."

Does it imply that Excel 2003 is little better than Excel 2007 on memory to
store the indexes to the increased number of rows and columns?
I work with a larger workbooks about 10 MB ~ 25 MB for office 2000 under
Window XP, If I upgrade to Excel 2003 / 2007, does anyone have any
suggestions on what the different in term of performance on calculation
speeds is about? Which one is performed faster on calculation? ignoring the
file size on memory to store the indexes, because I can change to larger HDD
over 250 GB.

Do you have any suggestions?
Thanks you for any suggestions
Eric