To get back on topic...
The 6 speed gearbox has been able to handle power just fine. A couple of people have had problems with actually breaking gears, but I would not consider that the norm. Those folks(like Alberto) abuse the transmission worse than even the average enthusiast.
However, the stock clutch will not hold up very long at all. In fact, the stock clutch won't hold up at stock levels if you drag race often. The clutch in my 05 lasted ~25k miles, but that included about 150 passes down the 1/4, all with full throttle upshifts and mid-rpm launches.
Also, since the intro of the HR, Nissan has had problems with the integrated slave cylinder within the transmission. Here's a thread on another site that may be of interest:
http://my[remove]350z[remove].com/forum/vq35hr/428616-bad-news-for-hr-owners.html