Well, there is no single best camera out there for everybody, but I'll give you some pointers at what to look for. Since you're used to shooting with an SLR and you are looking to take classes, you definitely want to go with a DSLR (but you probably already know that). Since you're looking at Nikon (I'm assuming because you want to use the lenses you have for the N65), the D70s or D50 are the cameras to look at. If your budget allows, the D200 is also a very nice camera. I would suggest going to a camera shop and playing with each of them and deciding which suits you best. I don't know with how much money you're willing to spend, but you'll probably be happy with either of those cameras. If I were in your shoes, I would most likely go for the D70s, unless I were richer and could afford the D200. But that's me. Also, check out the comprehensive reviews at
dpreview.com*. There you'll be able to read about pretty much every difference there is between those cameras, and see if there's some feature one has that another doesn't that you really want.
But basically, the more money you spend the better the camera you'll get. But that doesn't mean you'll take better pictures because of it. Any of Nikon's DSLRs will be able to take great pics, and you'll likely be better off investing in quality glass than a in a fancy body. But it's really all up to you. Go to a store and play with the cameras, read reviews, learn about them, and then make a decision about which suits you best.
* Review for the D70/D70s
here, for the D50
here, and for the D200
here. Also,
here's a quick side-by-side comparison between them
