I got the new 15" PB. 1.67, 512, 5400 rpm.
Whilst fidling around with Final Cut Pro, the PB get a bit laggy. What's the cause, the 512 ram or the harddrive?
Did you have anything else opened when you were using FCP? If so, try it with just that and when you do, open Activity Monitor to see your RAM. If you have a lot of RAM free then it's not a RAM problem. Tell us how much you have remaining (in green) while running FCP.