From everything that I have heard, grad school helps you a bit, but not too much. Schools will look at your UGPA only when making decisions - they might look at your MBA grades to see what you have been doing for the last 15 months, but it won't be a huge factor in your app probably. Your UGPA is pretty good though for everyone but the top 14 or so schools - where are you wanting to apply? The LSAT will be determining (as you already know).
If you score significantly higher, many schools will completely overlook your first LSAT, some if you score 4, 8, 10, 15 points higher, some look at the higher score no matter what, and some average no matter what - there is a thread a few pages back either on this board on the "where should I go in the fall?" board - do a search, it has pretty much every top 100 schools and then some, and their policies on multiple LSAT scores. If your first score was really bad (like below 140) and the school you really want to go to averages, then it might be better to wait for 3 years until the score is not valid anymore at most schools - you have already gone one year. A few schools, but not many, look at scores up to 5 years old - but it is rare and they would definaly take a significant increase in the last few years into account.
It is totally possible for an incease of 20+ points, many on this board have done it- you might want to consider a prep class (powerscore and testmasters are great, as is blueprint if you are in cali. I have heard) and definatly get the powerscore LR and games bibles, and do every prep test you can get your hands on.
Best of luck, and definatly ask if you have any questions, most people on this board are extreamly helpfull and honest (and those that aren't are usually pretty damn funny