I just upgraded from a 4mp sensor to a 12mp sensor, and my 2004-vintage computer is holding up just fine. Obviously things aren't as quick as they were before, but they're not agonizingly slow either. I have a 1.8 GHz AMD with single core and 2 GB memory. On the other hand, I don't use NX either - I use Photoshop.
Actually, the thing that's the biggest difference is simply how long it takes to download images into the computer, and that's really a function of the reader and card speed rather than anything in memory or CPU. And it's easy to just walk away while that's happening, too.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!