It will be based on the NT core, but it will have some backwards compatibility with 98 and stuff. This will make it a weightier OS than 2000 alone, I would imagine, and if you have a program like SOnar that runs like a champ on 2000, then leave 2000 unless you need all that backwards compatability.
One cool thing, though, MIGHT be that you can run SONAR on XP and get its stability bonus thanks to the NT core, and perhaps use plugins that were designed for 98 that weren't meant to run on NT. Maybe. Anyone?
Isnt XP supporting 64 bit also? If so, once the CPU's and motherboard catch up, you'll be able to get alot more processing done in the same time period that you do now.