Less than a stop and, in my opinion, nowhere near enough to justify the price difference. Also, some lens tests have put the f1.8 as sharper than the f1.4 (although you'd need keen eyes to spot it).
Originally Posted by eheldreth
I agonised over the same thing a couple of years ago, went with f1.8 and it hasn't disappointed me. Maybe consider putting the surplus into a set of extension tubes for close-up work (no additional glass to interfere with your existing optics, there's a 3rd-party manufacturer that does them with AF so your N80 will still TTL meter through them) or look at chopping in the 28-80 for a 28-105 (better optics, useful range and a semi-macro facility).
You might also want to ask around here http://www.nikonians.org