whenever lowering the car, you wanna make sure and put aftermarket shocks on too. The KYB GR-2 gas shocks are nice. I love mine, they help the ride a lot. Everyone here who has em swears by them. It's also best to do all at once when lowering b/c it's all part of the same assy. You'll save in the long run.
Most coilovers are the spring and strut assy in one and are adjustable up to a 4" drops or so on other cars (mainly Hondas). They run a little more than just springs, but still a drop for the money.
Then, there is the spring and perch assy only with your own struts (OEM or aftermarket). The perch is exactly what it sounds like, it's what the spring sits on. And that part is the adjustable part. The only coil overs for the Altima are this type, they're not a true coil over.