Easiest instant upgrade?
Intake. I think mine was $400. It isn't a big jump in power, but it is a step up...and when you are ready, contact nicoclub user RED_DET. He hooked me up with my intake. It's from R2C. It is simply awesome. I love it. I highly recommend it. A bit more power, and with mine, it has actually helped mpg a skosh as well.
Exhaust. Not one I have gotten yet (4 kids, house pmt., bike & car pmt's), but you free up the air coming in, and you certainly free up the air (or the used up air...carbon monoxide) going out.
My next upgrade will be an exhaust...and not some fart can s*** either!
IC? Upgrade the computer controlling the system. I haven't seen any evidence on an Infiniti, but I had a friend with an Audi A4, and when he chipped his, is was certainly felt with the "butt dyno".