First try the latest FireGL drivers:
Or simply download Omega Man's latest modded drivers:
(donate a few bucks to the guy for his work if it saves you from having to do the following driver modding)
If you really want to modify the Cat's to work on the FireGL, you will have to edit the INF and the INI install files.
For the INF files:
For the FireGL X1 128MB the device codes are as below, but for any other FireGL card, the device codes can be found in the corresponding driver. So for the X2 the device code to add to the Cat .INF file would be found in the corresponding FireGL .INF file and likewise with the .INI files.
In the 2KXP_INF folder, find where it says [ATI.Mfg]
Delete everything under that subsection and replace it with
"RADEON 9700 PRO" = ati2mtag_R300, PCI\VEN_1002&DEV_4E47&SUBSYS_01721002
"RADEON 9700 PRO - Secondary" = ati2mtag_R300, PCI\VEN_1002&DEV_4E67&SUBSYS_01731002
Do that with the C2_*.INF file if you have W2K and the CX_*.INF file if you have Windows XP.
Then you need to download RivaTuner.
Open it, click on the "power user" tab.
Click "ok" on that lame ass warning.
Click on the 3rd little icon from the left on the bottom that says "open Patch script".
It will open inside the RivaTuner folder, Click on the "PatchScripts" folder.
Click on the "ATI" Folder.
Click on the "SoftR9x00" folder.
Click on "SoftR9x00 w2k.rts" unless you're running 9x still...
On the bottom, make sure it says "force RADEON 9700 capabilities"
Browse into your catylist folder and look for the ati2mtag.sys file whever you extracted your drivers. (usually C:\ATI\SUPPORT)
That will allow you to manually install the drivers through Device Manager.
To install them through the setup utility you need to change the two .INI files CX_*.INI and INSTALL.INI (or C2_*.INI if you W2K) and modify corresponding OS section like so:
(0x4E47 being DeviceID and 0x01721002 the SubsystemID + SubsytemVendorID for the FireGL X1)
Do note that the last bit, the CX_*.INF should be the .INF filename you have just modded.
And that's a wrap, apologies if there are any errors in there, let me know if it works for you, if not I'll be happy to help.