Hm, is it really crashing right after it tries to load X, or is it just a case of a wrong xconfig? I've never used Mandrake Linux, but try pressing CTRL/ALT/F1-12 and CTRL/ALT/+ or - to see if you can get back to the console. The 490CDT has S3 Virge graphics - if you installed the S3 specific XServer try it the SVGA server instead. If all else fails, choose the VGA16 server, that's gotta work and you don't really need X for a firewall.
Oh, or ignore the above and look at what the search engine brought home

:
"Von:Erik Andreassen (Erik.Andreassen@xtra.co.nz)
Betrifft:Re: S3 ViRGE/MX + Linux Mandrake 7.2 (XFree86 4.0.1) = White Screen of Death
Newsgroups:alt.os.linux.mandrake
View this article only
Datum:2001-02-11 23:28:55 PST
"LD" <nospam@here.now> wrote in message
news:3a8727dd.1421288@news2.thot.net...
>
> Hello,
>
> Decided to Upgrade from Mandrake 7.0 to Mandrake 7.2.
>
> Everything goes fine except the X install.
>
> It seems the S3 ViRGE/MX Server isn't working.
>
> If anybody out there is using XFree86 4.0.1 or 4.0.2 with an S3
> ViRGE/MX chipset, if would be appreciated if you could let me know
> what you did to get it going.
>
> System Config:
> Mandrake 7.2
> XFree84 4.0.1 (tried upgrading to 4.0.2 with the Cooker RPM's, but no
> luck)
> S3 ViRGE/MX w/ 2048 KB RAM (Chipset 86C260)
>
> Toshiba Satellite Pro 490CDT w/64 MB RAM
>
> LD
simular problem and the reason --- default boot for 7.2 is framebuffer
enabled mode and guess which cards have problems with this.
do your install and do not do a test or run mode (it should finish and
reboot just after)
boot into failsafe mode
when it apears select configure (can't remeber the rest)
go into boot mode
select any of the boot modules
when open configure each ones vga to normal
back out and reboot
should fix the problem
annoyed me no end after the install to be presented with an black nothing
and to add insult to injury the monitor went to sleep :-("