if your going to use cad as a primary function, your looking at the wrong lineup of notebooks. div is on the mark, look for nvidia quadro powered notebooks. that type of notebook is generally called a "mobile workstation".
anything over 15.6" is going to see the price range go up and over $5-6k, so if your budget is at $4k, might need to consider the screen. the resolution on most mobile workstations are higher than the gaming notebooks anyhow.
ps: all the bickering over brands is lol. ultimately, its service you should look for in a notebook. unfortunately for the anti-apple tribes, they have the best service (apple). imo there is no difference in support from dell/hp/lenovo/toshiba, the only way youll get great service is with carepacks (that includes apple as well). asus have a slight advantage for people whom dont purchase carepacks, due to the amount of asustek service centres in australia (i think its 1/state). normally offer pickup/delivery to all metro areas and you can drop your notebook off at the centre.
i dont know enough about dell in regards to whether they use authorised repair centres or have their own, but generally (in order):
apple (combined; apple genius bar, authorised agents)
asus (combined; asus service centre, authorised agents)
toshiba (combined; toshiba mobilecare/procare, toshiba authorised service providers)
samsung (combined; samsung service plaza, authorised agents)
hp (authorised agents)
lenovo (authorised agents)
--
dell (?)
anyway, gl!
ps bozo: everything in our office is microsoft, but cant knock the osx environment for what most mac users operate in (or can you?? ). i cant navigate the osx ship like i can the windows/server ship, but the old man has a mac and i pissfart around with the ios4 sdk off of it; nice light os.