Here's a link to help you get started:
http://www.littleitalynyc.com/about.asp
I've eaten in many of the restaurants by my favorite, by far, is Angelo's of Mulberry Street. The food is very well prepared and the atmosphere is genial. It is quite expensive but worth every penny.
I've also eaten at Il Cortile, Luna, SPQR, and Costa Azurra. The Original Vincent's and Grotta Azurra. None of them touch Angelo's. If you are on a budget, though, Luna is probably the least expensively priced of the group.
For dessert, a trip to Ferrara's is a must. The cafe is lovely, very busy and the pastries are irresistable. Take home a box if you have no room after dinner. Some people also love the sfogliatelle at La Bella Ferrara (and I have to admit, they are quite delish) but unless you are Italian, you probably wouldn't order that anyhow.
Chinatown is right around the corner and I'd visit that area as well if you are in NY for the day.
Oh, and reservations are a must at many of these restaurants and mandatory for some. I'd call a week in advance just to reserve.