Stickers are definately important to improving performance :POn a serious note i'm guessing that maxima has a VQ35DE in it? If so you will get very little power gain though you may get a bit more response, for an N/A 3.5L you really don't need that big of an exhaust to get maximum power out of the engine, header design does come into play quite a lot but from the pictures i've seen the standard headers aren't THAT bad for stockies. Overall its a waste of money unless your just trying to burn money.
If your car had a VQ30DET in it then I would say definately do it as well as a new dump pipe and cat which will mean FAR less back pressure on the exhaust turbine meaning it will spool faster and be more efficient thus generating more usable power and torque at lower revs.
The ideal exhaust size for turbo engines is quite different to N/A as in the case of a turbo the exhaust is more concerned about getting the gas from the back of the turbo out of the pipes where as in N/A platforms exhaust sizing can greatly affect torque curves in conjunction with header design and exhaust piping of too great a size and have negative effects on performance in some cases.
On a standard VQ35DE I can't see you getting anymore than 10-15whp more out of it on a standard tune. If you want it just for the sound i'm sure you can get a half decent muffler much cheaper that will give you the noise you want.
