I've read mixed reviews about HKS exhausts, but they certainly look the business. There was a video on YouTube of a UK 370Z owner who had the HKS Super-Sound system fitted by Abbey. That sounded good. But for the money, and considering the potential hassle of sourcing one over here, (not sure that Abbey even supply them any more - I think they've moved over to Milltek) you'd be better off going for one of the more readily available systems. There's a ton of choice - check out the 'Tarmac Sportz' website, for example.
http://www.tarmacspo...47-09b52eb6090b
Be aware that there can be minor fitting issues when fitting an after-market exhaust to the Nismo. They're usually fairly easy to solve though.
You might want to have a look at my thread about fitting a Stillen exhaust and Cobra high flow cats to my Nismo (definitely well worth it, despite being quite pricey). There's a brief sound clip at the end.
http://www.350z-uk.c...llen-treatment/
The Stillen is a great choice - even without the high flow cats it sounds really good. It also has the advantage of looking similar to the stock Nismo system when fitted. One or two of the popular systems, such as the Invidia, might not look as good on the Nismo as they do on the other 370s, due to the design of the exhaust tips. I think the Nismo needs quite hefty looking fully round tips to fill the rear diffuser or it might look a bit odd.