I have a D50 and I love it. I bought it last April. The 70-300 you have is prob the same one I have. It is a decent lens, and I have used it a lot at Carolina Hurricanes games, and did not have too big of an issue with it. Yes it is a slow lens, but I know Hockey so I wasn't just holding the shutter down as long as I could and hoped for the best. You should be fine with it for Lacrosse games, most of them are prob during the day. Don't be afraid to crank up the ISO, it is very good at 1600.
If you have any other questions, just ask, and if you click on my sig, there are a lot of Nikon users on that site.