you can do whatever you want... the resale value and versitility of the D70 is a major plus and you can get one for less then the D40 you're looking at. SO it's win-win. Chepaer, and more versatile if you get into photography... at the same time if you dont like it you'll be able to sell it easily.

Dont get fooled dude, if you dont like it as a hobby there is just about no way you'll still coninue using the SLR as a every occasion camera. too heavy and bulky.

as for the difference i dont know the D40 that much. i just did a little serch and heres what IMO are differences:

-1005 points light mettering on the D70 instead of 500 on the D40.

-D70=1/8000 sec while the D40 is 1/4000

-D70 has a 5 point AF system as opposed to 3 points for the D40

But 1 advantage the D40 has is it's smaller and 10MP. So if you are goingt o do alot of enlargements maybe you'll see a difference.