Memory limit and execution time

Hi. I’ve just been on to my host (Hostgator) and they have increased my memory limit to 64mb as required however it is not reflecting this on my diagnostics page even when I reset in the backup screen. They changed the memory limit in the PHP.ini file.
They also told me that the execution time can not be changed unless I have a VPS server. Will this make a big difference as I don’t want to pay additional hosting charges at this time.
Also one more question: What is the benefit of installing the demo files? Is this necessary? or does it make it easier to set the theme up?

Thanks

Sue