Excel - Insufficient Memory, Freezing Up, Slow Recalculations

Version: 2008
Operating System: Mac OS X 10.5 (Leopard)
Processor: Intel

In May 2008 I bought a new MacPro (4GB) with Mac OS X 10.5.4 and installed
Microsoft Office 2008 including Excel 2008 for Mac (now 12.1.1). I have
transferred spreadsheets created on a Mac G5 under the old Excel 2004 for
Mac to the new MacPro and have also created new spreadsheets. With both old
and new spreadsheets, I am getting notices of insufficient memory, freezing
up of Excel and the loss of input since a prior Save, and noticeably slow
recalculations for large spreadsheets. I had no problems of this kind on
the Mac G5 with the 2004 version of Excel. What are possible solutions to
this problem? Is it time to migrate to Apple's Numbers program?