How much should my girlfreind and I plan on spending in nyc for a three day weekend. Airfare is paid?

It really depends on what all you plan to do while you're there. Shopping? Sightseeing tours? I was there in 2004 and 2005 for a weekend and not counting the airfare or lodging I spent less than $500 and that was eating cheap, one boat tour of the Hudson which I would highly recommend because if you've never been there its a great way to see where everything is. You can see the Brooklyn Bridge, statue of Liberty, Ellis Island, etc, all from the water. The best thing to do is plan a budget and check out the best NYC travel website and figure out what you want to do while you're there. I personally loved hanging out in Times Square and strolling through Central Park on a horse and carriage ride.