I'd recommend Dell, but no matter what mfg you select, MAKE SURE you pick Intel Core 2 Duo processor (speed depends on what you can afford, but 2.33GHz is really nice). Core 2 Duo processor just kicks ~~~~~. It doesn't make any sense to buy a laptop today that doesn't have Core 2 Duo, as there isn't much price difference. Max it out with 2GB (or more) of DDR2-667MHz RAM and pick a fast 100GB 7200RPM drive - both also highly recommended.
Those above would be the specs to go by, everything else is just whatever you want in it which don't make much difference in a DAW, but I'd pick a dedicated graphics card with 256 or 512MB of Video RAM, not one of those Intel GMA shared graphics cards which use main memory for video. Laptop with those specs will approach a desktop in performance.
Your RME box may not like the laptop's built-in firewire port, but if you get any trouble you can use a PC-Card firewire adapter which would solve those kinds of problems (like pops and clicks in audio). Some laptop firewire ports don't play along that well with external audio cards.
I was thinking of getting a laptop with core 2 duo so I can unload my studio computer with the sort of use that is not studio use. However I am considering making the laptop into my studio computer instead since it will be much more powerful than the computer I already have.
Now the tricky part is to use two screens with a laptop. Is there any laptop/gfx card out there that can handle two external screens (and perhaps + the built in screen)? Because that is something I prioritize in a studio computer.
I am aware of my options Core 2 Duo, Dell, Mac, etc. What I really need to know is how stable are these laptops to run the apps and devices I listed? I don't want to spend money on something that will not work.
As long as you properly configure the laptop (dedicated video ram, fast processor/ram/hard drive, etc) you won't have a problem running anything you listed on any of the new laptops.
I've been using Dell's for a while because if you wait to get a coupon code they're by far the best bang for the buck. One thing to watch out for with the new Dells is that they don't use PCMCIA slots anymore, they use PC Card Express. So if you want to buy a firewire 800 card to use your Fireface 800 to it's max potential you'll have to make sure there's a firewire 800 PC Card Express card available.
Speaking of firewire 800.. the new Macbook Pros do look very nice, you'll be able to run OSX and XP. I'm going to take a long look at them for my next upgrade..