I would suggest something with:
i7
8 GB memory or higher
17.3" screen - laptop would likely be large enough for 2nd internal HD.
Any internal drive should be 7,200 RPM or SSD (I recommend 2 SATA 3 drives - primary 500 GB - 1 TB, secondary drive 1 TB to 2 TB)
If you have the coin, then as large of an SSD for primary drive as possible - but the SATA 3 7,200 RPM non-SSD drives are plenty fast enough for Sonar. I used to have a primary drive that wa SSD, but when it failed I replaced it with a non-SSD drive and had enough money left over for a nice new 46" HDTV, which is now my primary monitor.
You certainly will want at least 1 USB 3.0 port on the laptop, try for at least a couple of those, with one or more USB 2.0 ports as well, in case your audio interface needs a USB 2.0 port.
If you want a 15.4" screen, you will likely get a single internal drive, which is still OK, as long as you have either a USB 3.0 port or eSATA port for an external secondary drive. Getting the larger 17.3" screen sized laptop would allow that secondary drive to be internal, and would be a bit less cumbersome for mobile use.
Here are links to a few - some may not be in stock, I just chose these for the specs and relative cost - shop around for price and availability:
http://microcenter.com/pr...Computer_-_Black_Widow (about $1250 - has two 7,200 500 GB drives and 12 GB memory, 1 open slot for an additional 4 GB memory totaling 16 GB)
http://microcenter.com/pr...puter_-_Midnight_Black (this one is about $1000, but has a 5,400 RPM drive - you would want a faster 7,200 drive - do not know if room for 2nd)
http://microcenter.com/pr...aptop_Computer_-_Black (about $1200 - comes with 7,200 750 GB drive and 8 GB memory - 2x4GB - upgradable to 16GB, but you would have to scrap the existing strips and get 2 8GB strips which would be expensive)
Anyways - there seems to be a bunch of machines in the ball park of $1000 to $1300 that would be quite powerful for several years of mobile use. You can run Sonar on lesser machines, but I believe the difference in performance between what you would get in a laptop for $600-$800 and one running an i7 with a combination of lots of memory and faster drives is well worth the extra couple of hundred or so to get the more powerful machine.
Sonar loves power and memory - and having 2 drives is also the way to go, in my opinion.
Bob Bone