You can find a lot of good rims for $800 or less. But you're going to need buy new tires, and it will set you back quite a bit more.
Check out this site; they usually have good deals on wheel/tire combos.
http://www.wheelsnext.com/wheels-tires-packages.html
Make sure that you get the 5x114.3 lug pattern.
Other things you'll come across are offset, which determines where the mounting bracket is - in millimeters from the center of the wheel. The width of the wheel will determine what kind of tire you need. Also the center bore may not be the same size as the part you mount it on. To avoid vibration, I'd recommend getting hub rings.
This thread should help you some:
what-wheels-will-fit-my-altima-will-it- ... 46628.html