Hello Sanssan,
Since you have the 32-bit Windows 7 installed, you're system will only use up to 3.25 GB of your available RAM. You would need to install the 64-bit Windows 7 instead to make full use of all of your RAM instead.
It's usually best to allow Windows 7 to use that default
Automatically manage paging file size for all drives setting for
Virtual Memory (page file) unless you have separate physical hard drive that you want to have the page file on instead for better performance.
Hope this helps,
Shawn