To change the maximum template size to 500mb open a command prompt and type;
stsadm.exe -o setproperty -pn max-template-document-size -pv 524288000
Note if stsadm.exe isnt in your path you might have to navigate to
C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN
Dont be frightened if the command chugs away for a while.
