Yeah, just hand edit the file, no worries. You might want to try to put in a line called: DefaultDepth near the top of the display section, like:
Identifier "Display Merged"
Device "MATROX CARD 1"
Monitor "Display Merged"
Option "Monitor2Position" "RightOf"
Option "MetaModes" "1280x1024-1280x1024 1280x1024 "
Option "Monitor2HSync" "31.0-69.0 "
Option "Monitor2VRefresh" "50.0-160.0 "
Virtual 2560 1024
Hopefully that's not a confusing example, ignore the virtual part, dual-head card... Also, I'm using a voodoo3 with Slack 9 on one machine and I think they made some severe mistakes in the driver as it always caused my machine to go wonko kazoo with XFree86 4.3.0. They added a ton of support for older 3dfx cards so that might have something to do with it. If you upgrade one day and get lots of artifacting and random screen lockups, post back here and I'll show you how to straighten it out, or maybe they'll beat that with XFree 4.3.whatever's next and RH or your next distro of choice will skip 4.3.0.