I wasn't able to burn CD-R's at 48x with my Nec 3500a with Nero 6.6.0.8 with firmware 2.18. It listed 48x originally then when I went to burn at 48x the first time I used Nero 6.6.0.8 it knocked it down to 32x saying "cannot burn to media at 48x". After it did that it wouldn't even list 48x as an option anymore in future sessions.

With firmware version 2.19 Nero 6.6.0.8 lists and writes at 48x now. Yahoo.

http://support.necsam.com/oem/downloads/flashfirmware