There are a few areas around there. Northgate Park is to the east of the mall, and is a pretty nice neighborhood. A little bit south of Northgate Park is Duke Park, and that's a great neighborhood as well.
When you say the area directly north of Northgate Mall, you mean between Guess Rd and I-85? A lot of those apartment complexes are not as desirable. There's also a small neighborhood (Hillcrest Rd, Forest Rd), but there is always a lot of freeway noise there.
I'd personally stick to closer to Northgate Park or Duke Park.
Also, check out this website for some info on different neighborhoods. There's also links to each neighborhood's listserv, where you can inquire about rentals:
Durham Hoods - Maps of Neighborhood Mailing Lists in Durham, NC