The answer to buying a laptop (or any computer) is realtively simple; buy
the best that you can afford!
Neither N6 or Nvivo are particularly power-hungry applications and any
current spec laptop should be fine.
It's more important that you consider quality/reliability of the product,
how you will use it (if you travel a lot, focus on light weight; if you just
move from home to office, maybe that's not the big issue). What about
connectivity (to a network, wireless, Internet access - how, when, where?).
How will you back it up (if you're not networked, consider a CD-RW as
critical!). Have you used laptops before and liked or disliked particular
things about them? Is battery life an issue or will you be on mains power
most of the time? Also consider the warranty and the suppliers reputation.
Too often we focus on the technology but 9 times out of 10, the real issue
is what we want to do with that technology; what will work *best* for me :-)
Hope that helps!
BTW, I use Nvivo on my laptop (Dell Inspiron 8200) and it runs fine, no
different to any of my desktop PCs.