I'm sure this is caused by lack of RAM as premier pro needs at least 512Mb and preferably more to run smoothly. Try more RAM again and make sure it is the same type (or compatible with) your existing RAM and Mobo. Other than that maybe your hard drive isnt up to scratch - try defragging it and see how you go.

In premier ensure your project settings are the same as your imported footage - pal/ntsc etc.
Your processor is plenty up to the task so apart from whats been written I cant see what else to do although I'm certain its a RAM thing.

Tonga