Premiere Pro will use what RAM it needs to get the job done quickly. This is a good thing, and is one of the benefits of being a native 64 bit program.
You shouldn't be using your edit computer for 'anything else' anyway.
i got premiere pro cs5,5. My operating system is windows 7 home premium, 64 bit. i've got 12 GB ram with a intel core i5- 3540 CPU 3.10 GHz. all in all, a above average computer build for gaming. the problem occurs when i i try to play a importet movie in the programwindow, and when i eksport a movie from Premiere. ram usage goes from 20 % to 80 %, and everything on my computer starts lagging a bit.
Do not do anything else while editing video
What are you editing, and does your project match your video?
Read Bill Hunt on a file type as WRAPPER http://forums.adobe.com/thread/440037
What is a CODEC... a Primer http://forums.adobe.com/thread/546811
What CODEC is INSIDE that file? http://forums.adobe.com/thread/440037
Report back with the codec details of your file, use the programs below... a screen
How many hard drives, and do you have your projects and video files on a separate hard drive or do you only have one hard drive?