This really depends on what hardware/software you want to use. For Pro Tools the best performance is coming from Athlon XP's for desktops and Centrino processors for laptops(the centrino's are actually out peforming desktops right now!). The best advice I could give is to figure out what software/hardware you want to use and then see what computer configurations are working best with it.
I myself have a XP based laptop that manages 32 audio tracks(max for PT le) w/ 6 aux tracks running 190 fx plug-ins. Now as an example of the different kind of performance you can get with different setups I can also get 48 audio tracks w/ 240 fx plug ins in Sonar 3 on the same laptop. These are just torture tests I run to test the limits of my gear, needless to say I would never actually use that many plug ins. Our main setup is a Mac G4 dual 1.25 w/ digi 002 that ironicaly is only capable of 27 audio tracks w/135 plug ins, it handles "real world" sessions well enough though. As far as what hardware to go with, I'd recommend a firewire interface as opposed to a USB interface(USB is finicky and somewhat limited compared to firewire).
I'd suggest downloading demos of all the top multi-track software and trying them out to see what works best for you as some of these programs are easier to use then others but of course it's really all subjective to the person using it. Also the websites for the hardware/software companies usually list what computers are working best with their equipment.
Im also looking at a Motif ES6.Rack sampler,virtual or sampler keyboard
The Motif ES is all of those, I just gave mine to my cousin for his birthday but it's a great machine, tons of great sounds, a gig of sample ram, crazy re-sampling features, tons of fx, USB storage and with the mLan option a 14 channel audio interface. I was gonna get another one but decided on another MPC 2000XL, E-MU modules and Korg MS2000(I needed a new "home" setup cuz we just moved all of our equipment to a new spot, no more luggin' gear back and forth thank god!) A lot of people think that the Motif is a pain in the ass to use but I found it easy enough except for the file arcitecture, they set that up ass first as far as I'm concerned.
Just take your time (prices will only get lower) and do A LOT of research! You also might want to wait for the winter NAMM show, a lot of people think that Korg is going to introduce a new workstation which means prices could drop even more. Good luck!