pretty sure a 324 will require a 17" wheel.
a rotor upgrade will be heaps better than the 280mm because braking is a function of
1 - torque that is applied to the rotor (increased rotor diameter, higher torque)
2 - rotor's ability to dissapate the heat (larger rotor, more mass, more heat capacity)
3 - caliper itself and ability to consistently provide an even force across the face of the pad
consider that the factory 4 pot nissan caliper is a pretty good thing, just that the rotors are a bit small for dissapating track heat. go the 324s and you will find that you might not even need a caliper upgrade. what may be of worth is a caliper rebuild kit (all the rubber seals) and a bit of time to rebuild the factory caliper... don't think the seal kit would be more than $150 at a guess?