Hi,
The trouble with laptops is that there is always the possibility of bottlenecks that you can do nothing about - i.e, one component stopping another reaching it's full potential. A mate of mine bought a new laptop with a C2D processor, all singing all dancing machine, but it was nowhere near as good as my old single CPU P4 laptop.
I've just posted my spec on the "PC - vfm?" thread. A mate of mine with the same spec gets 30 frames per second (locked at 30) on FSX constantly. Have you already bought the laptop? If not, and mobility isn't an issue, I'd go for a desktop. If you've got the laptop already, I'd definately put an extra Gig of RAM in.
Just to throw a spanner in the works
Cheers
AV8