You can change the amount of levels Excel remembers for Undo.
The default is 16.You can increase this by navigating to :

- HKEY_CURRENT_USER\Software\Microsoft\Office
\VERSION\Excel\Options\

Note : VERSION = 9.0 , 10.0 , 11.0 , 12.0 (?)

Select or create here in the right pane the entry :

UndoHistory

(right click in right pane , new , REG_DWORD [DWord Value]
and name it as above)

Doubleclick this entry and set it1s value to that amount of levels
you want to have remembered for Undo.
Close registry , restart Excel!

Note:I tested this tweak in Excel 2003 and it worked fine.I checked the registry
on a PC running Office 2007 as well.I did not find there an Options subkey
under the key : ..\12.0\Excel\.So i just can say it works on Office 2003
and propably older versions.But feel free to create the missing keys from the
path above with Office 2007 installed and test it.