Around ET20 works well... Moreso in 19" fitment...
ET20 examples can be seen here - http://www.350z-uk.com/topic/32711-rota-gtr-wheels-%e2%80%93-350z-fitment-from-cougar-store/
We can sell just the mid and rear silencer, which Y Pipe do you have fitted at present... Ideally the Milltek works well as a cat back system, an option maybe to sell your current Y pipe and then purchase the full Milltek system...
We had a Milltek alongside an Invidia, the Milltek was noticeably quieter but it all depends on what you want from a replacement exhaust, many people love the Milltek as a step up from the OEM system twinned with its excellent build quality too...