> I just installed 3.0.1 and try to get X working at 1024x768. So far, I 
> get 800x600. When I try "X -configure" it segfaults in XF86Config.core. 

I have no specific knowledge about amd64 or XFree86, but you asked for
hints....

ISTM that you found a bug, by definition (unless XFree86 is documented to
segfault). I would guess that the -configure option is tripping on
something it didn't expect while it's trying to discover what you've got. 
You might consider reporting the bug to the XFree86 (or X.org) project. 
They'll want the hardware details and a backtrace of the coredump.  

Meanwhile, if your driver supports your hardware at that resolution,
you'll have to set it up without it.  

I'd start with your hardware and the driver.  Your dmesg(1) output and
your X logfile (/var/log/XFree86.0.log, perhaps) should show your hardware
details.  Then check the documentation for the driver to make sure it's
supposed to support the video mode you're hoping for.  Let's assume you've
done that, since you probably have.  

The X log should show the all the modes it tries, and which ones were
discarded, and why.  If 1024x768 was discarded, you're out of luck unless
you can figure out why and change it.  Of the ones it kept, you probably
know you can rotate throught them with ctrl-alt-[+-] or with xvidtune.  

If you can get the thing into 1024x768 but not by default, then I do have
a helpful hint.  See your "Screen" section.  Mine looks like this:

Section "Screen"
        Identifier "Screen0"
        Device     "Card0"
        Monitor    "Monitor0"
        DefaultDepth 24
        SubSection "Display"
                Viewport   0 0
                Depth     1
        EndSubSection
[...]

The "Display" section can hold quite about a bit, including what
resolution(s) to use.  The default "Display", however, is driver-specific,
which is why "DefaultDepth" is handy.  By using the depth to identify the
display and the display to capture the resolution (etc.), you can coerce X
to default to your favorite settings.  

Sorry if this was obvious.  Hope it helps.  

--jkl