In my opinion (if you're going Nikon) the D80 is the lowest you should go unless you are trying to save money. If you simply cannot afford the D80, then fine... go D60, and if you can't swing that, then honestly I'd suggest buying a used D70S. I could not in good conscience recommend a D40. I won't go quite as far as Bi, but only because I'm trying to be fair to people who don't want to (or can't) spend the cash. They're "fine", but I don't recommend them.
If you're a first time user of a DSLR, I recommend buying a single flexible inexpensive lens to start. I am very fond of the 18-135 that comes as a kit lens in many cases. The D80 with this lens is like ~$1000 total.
This gives you a good lens to start working with, will give you some good flexibility to take a variety of shots, and will give you some clear walls to bump into and make some mistakes. (you learn a lot from mistakes)
This also stays away from spending $1000 on a lens that you eventually realize wasn't the one you need... so when you wind up retiring that 18-135 you won't cry yourself to sleep at night.
People will say on and on that you gotta buy the best glass you can... and ultimately I agree, but initially you wouldn't notice the difference between good glass and "not so good", so save your pennies for now and pickup some other accessories you WILL need much sooner, such as an external flash.
FYI, I use a D300 currently... I have used a D200 extensively, also own a D100 and have a fair amount of experience horsing around with the D80, D60 and D40. I have used the D300/200/100 for professional and personal work. I've put about 6000 shots through my D300 in the past 4-5 months.