I am now running the 5540 beta and OS X 10.5.1 on my MBP C2D 2.4 w/4GB. The memory footprint using this configuration appears improved to me with consistently more free memory available than under 5160. Impact on CPU also appears to be lower. Previously I would only run Parallels when I needed XP, but given the current performance I am finding that I have no qualms about keeping it running all the time. Under Leopard using Spaces I have one virtual desktop dedicated to my XP VM in full screen mode which is a very helpful feature. Parallels keeps moving forward and while there are some nice things in Fusion 1.1, the fact that you still can't print to a network printer using NAT is a show stopper for me. Still sticking with Parallels.