Hello Amadeus!

I'll try to answer what I understand from past researches and experiments.
first is you can do man options. Descriptions of entries for your kernel
config are there.

> a) I don't know what they do
> b) I don't know if I need them
>
> Documentation is a little scarce; although only looking at the
> netbsd-guide is perhaps a reason for this.
>
> maxusers        64              # estimated number of users
>
This controls the size of certain limits and statically allocated tables
inside the kernel (Number of  Maximum processes & processes per user,
limit of opened files for system and process)

> Also
>
> options         MATH_EMULATE    # floating point emulation
> options         VM86            # virtual 8086 emulation
> options         MTRR            # memory-type range register syscall
> support
>
If youre using processor Pentium 2 above its safe removing or processors
lower (133/166) that have FPU. Try to check your dmesg output and see if
those are present on the line "cpu0 features".

> I'm not sure I need these are; what does floating point emulation do?
> Is it needed?
>
> options         INSECURE        # disable kernel security levels - X needs
> this
>
> Seeing as I'm not using X would this cause problems if disabled?
>
If youre not going to run X, you can also remove this.

>
> options         SYSVMSG         # System V-like message queues
> options         SYSVSEM         # System V-like semaphores
> options         SYSVSHM         # System V-like memory sharing

If youre going to programs that uses System V like implementations like
Apache, Squid....  youll need those.

> How can I find out if the system is using things like
>
> pseudo-device   md              1       # memory disk device (ramdisk)
> pseudo-device   vnd             4       # disk-like interface to files
> pseudo-device   bpfilter        8       # Berkeley packet filter
> pseudo-device   sl              2       # Serial Line IP
> pseudo-device   strip           2       # Starmode Radio IP (Metricom)
> pseudo-device   irframetty              # IrDA frame line discipline
> pseudo-device   tun             2       # network tunneling over tty
> pseudo-device   gre             2       # generic L3 over IP tunnel
> pseudo-device   gif             4       # IPv[46] over IPv[46] tunnel
>

Try reading your dmesg output, tun & bpfilter will be useful if youre
using that box for gateway/firwall purposes (pppoe stuff)

> ? I know gre is used from looking at tcpdump from time to time, but
> otherwise I wouldn't know where to look.
>
> Sorry for the ignorant questions; a quick amazon search didn't reveal
> any netbsd-tweaking guides and I have always wondered about these
> things anyway.
>
> I guess my main question is: "how to find out what kernel options your
> system is using".
>

The tool adjustkernel this might also be helpful in tuning your system
its located at /usr/pkgsrc/sysutils/adjustkernel

HTH,

Joseph