Fishing can earn money, but fruits from another town can add up after you get them going. That is how I made most of my money for Animal Crossing on the GC.
I just got the game today, well yesterday actually, but I didn't really start playing it til today. I really haven't bought much from nook, however I have sold ALOT to him (enough to pay my house off and have around 5000 left over). He has allready told me he is upgrading tommorrow. Does selling to him help with upgradeing nooks maybe?
Only thing bad is I still need a axe and shovel and I can't buy it tommorrow lol. I got some oranges from Klouds town I want to plant.