Windows XP Pro Memory usage

I have just upgraded the memory on my notebook from 512 to 1GB to improve performance but it has made no difference. I have done all the normal stuff… defrag, disable paging executive, enabled LargeSystemCache and cleaned prefetch. It appears that there is a lot of free memory and the pagefile is being utilised.

How can I force applications to remain in memory and only page if necessary as this must surely improve performance?