As it has been written in every possible corner of this forum, in every relevant thread and discussion, on every wall, floor, roof, scrap pieces of paper, on the back of people\'s heads, on toilet seats, tabletops, billboards, mailboxes and desktops... and even the sky... lag is a random problem for everyone.
Your system specs are almost the same as mine, yet I have trouble keeping my framerate low enough so the CPU doesn\'t overheat and freeze my whole system.
Lots of RAM helps- you have plenty. Thus my guess for you would come down to your Radeon card. PS seems temperamental with some of those.
*hugs her GeForce FX lovingly*