The Core2Duo in the MBP will most definitely be faster than the processor you currently have in your Dell, especially since you'll be using a program where you can actually notice a difference.
I have a 2.0 GHz Macbook with 2GB of RAM and I can run Parallels quite smoothly. I used to run it when I had 1GB of RAM and while the machine slowed a bit when Windows was loading in a Parallels window, it was relatively smooth afterwards. Now with 2GB, Parallels loads without a hiccup.
I have tried both XP and Vista in Parallels and I stick with XP only because it requires fewer resources to run smoothly. So I can allocate just 512MB of RAM to it and it'll run just fine. Vista, on the other hand, wouldn't run as smooth with that amount of memory. 1GB would be ideal, but I've managed with 768MB (only because Aero doesn't work in Parallels).