First.. yes the RaQ2 , RaQ3, RaQ4 and XTR all make great nameservers and can run both current 8.x and 9.x versions of bind.
You should consider the OS (i.e running at least the 550os or better) for security reasons.
This is not longer true. There are to many exploits out for it that allow root access and I don't think i will be making updates for it since I never found it to be stable(gui) or data safe (xfs)... anything is better...
CentOS 4 is based on Redhat Enterprise Linux (RHEL) 4, which is a Linux 2.6.16 kernel
It's important to note here that the 2.6.16 kernel won't be quite as fast as it's little brother that contains only what you need for your 512 max 450Mhz cpu. So when building nameservers I suggest you use the smaller faster currently supported original kernel...
even if you are only running DNS? (No gui, no mail, or http reponses etc...) For security reasons as stated above wouldn't the BQ option be more secure?
Yes, the CentOS+Bq option is your best bet, as you have access to regular updates from a non-commercial source, and you have a modern OS and other software components. You will have to do some minor mods to shut off the other services completely (the web interface can't stop all of them entirely).
Anyone who tells you that an older RaQ OS is better is trying to sell you something, directly or indirectly. The older Raq kernels and software have numerous security bugs. I have no experience with Gentoo, so IronFist might be right about that, but it will be somewhat more roll-your-own.