I've made very good looking 20"x30" prints from my 6mp D50. I've had several made by Sam's Club. The price is right and they look good. Art gallery good? Maybe not. But good enough to impress your friends. With the D3000 there's almost no limit to how big you could enlarge, because as the enlargement gets bigger people are standing further away to view it.
I'd say experiment by going with Wal Mart (never tried them, but they are probably like Sam's Club) and if you like the results, you can pay more to get them done at a professional lab to look even better. But if your experience is like mine, you'll be happy to display what you get from Wal Mart.