replacing intel-agp.o

[Steve Kleene <skdeb@syrano.acb.uc.edu>]

The i810 video driver supplied with Etch isn't working too well with my
onboard Intel 946GZ video chipset.  There are errors in Xorg.0.log, Xvideo is
not supported, and xvidtune doesn't work.

I read here:
  http://lists.debian.org/debian-kernel/2007/01/msg00798.html
how to patch intel-agp.c for the 946GZ chipset but don't know enough to pull
this off. So far I have

1. downloaded the kernel source, including intel-agp.c, into a directory
   /tmp/tmp/linux-image-2.6.18-3-686/linux-2.6-2.6.18.dfsg.1
2. isolated the patch script from the post listed above
3. applied the patch manually (by inspection) to created the modified
   intel.agp.c.  I got errors when I tried the patch program, which I have
   never used before.

Now I'm not sure what the minimal way is to compile intel-agp.o.  I don't
want to overwrite any current system files (other than intel-agp.o).  I just
want to make intel-agp.o, put it in place, test it with modprobe, and see
what happens.

So what are the minimal commands I need to just make intel-agp.o?  Do I need
to put the whole source directory in some standard place, run make config,
and all that?  I've compiled many times over the years but have never gotten
into kernel source before and want to be careful.

Thanks.