I have a 2.0 GHz base model aluminum macbook. With the stock 2 GB RAM, OS X definitely feels sluggish whenever I run VMWare Fusion + Win XP. Today, I finally upgraded to 4 GB RAM. What a difference! Now I cannot noticed any lag what so ever: I'm converting a movie with Handbrake (OS X), loading photoshop in Windows and loading Word in Mac all at the same time. I can now run 5 or 6 (or more) apps at the same time, and they all feel fast as if I am just running a single app. It's just amazing! If you haven't done it, you should really consider upgrading the RAM.