I heard from someone that you can program the 03-04 350z map onto the 02-03 Maxima's ECU, how true is this, if it is how much more HP can I expect. or better yet I would I be able to swap a 03-04 350z ECU into the car.

I'm not too familiar with Nissan ECUs, but swapping ECUs is not plug and play.

With all of the electronic anti-thieft measures, the ECU must see certian things before it will let the car start.

Most ECUs have to be programmed to the car. Again, not being too familiar, there may be certain codes in the instrument cluster that tell the ECU its in the right car. There can be key data as well. Even swapping in an ECU from another Maxima may not work due to these anti-theft features.

If it was easy, everyone would be doing it.

you take your car to the nissan dealership and say "good sir I would like the 350z tune for my maxima" then they will bs you that its not possible A FAT LIE or they will do it for you and charge you like.. a couple of houndred bucks expect to pay around 350-450 if you can not find any local dealerships around willing to do so then call up the boys over at TechnoSquare and they can tune your ecu to your mods

would this work with a stock maxima.

yes