My base system description is important as the problems and solutions I describe here are only really pertinant to this system, and like systems.
The RME Hammerfall CardBus + Multiface works grewat under Linux 2.6, and proabably even better under linux 2.4.x. All I did to get it working was to compile the 2.6.4 kernel and turn on the PCMCIA options for the CardBus. Its not labeled RME CardBus or anything, but its not hard to find within xconfig. The real trick is in getting audio to come out of the device.
Once the kernel discovers the hardware, you need to load the firmware. Go to alsa-project.org and download their utils and tools. You dont need the drivers, as you built them into the kernel, right? (alsa section of kernel, but dont worry if you dont see the RME there, just select the drivers pertaining to the cardbus, and in my case the pci device Maestro3, as my laptop has said device)
Once you build the tools, youll find the hdsploader and hdspmixer programs have been installed. Run the hdsploader program and the output should say something like "found multiface", and there will be much rejoicing.
Now run hdspmixer and turn up the levels of output, and then save these settings as your default. youll have to do this combo of hdsploader, hdspmixer every time you boot your machine.
The alsa drivers dont seem to work directly, but jack on-top of alsa (jackd -d alsa -d hw:1) seems to work just fine. Also, the card works great in oss-emulation, which is how you get xmms and gxine and other programs like that to work.
Thats all there is to it. If you have specific questions I will answer them, if I can, and post them here as well.
I had some real problems compiling the standard kernel (the seccond time, I dont know why it worked without problems the first time...). It turns out that RedHat, the co-makers of Fedore Core, have some special kernels, and that you need to forget about that to get the standard kernel to work.
First download the kernel from kernel.org, and if you want, apply the patch I have here. Then run the following commands: make clean; make defconfig; make xconfig. After thats done you should have a windo where you can configure your kernel. Do so.
After you configure you kernel, run: make; make install. Now you should have an installed kernel. Run: df -h; and record where your root partition is. Mine is /dev/hda3. Now edit /etc/grub.conf and change kernel /vmlinuz-2.6.4 ro root=LABEL=/ rhgb to kernel /vmlinuz-2.6.4 ro root=/dev/hda3 rhgb. Save and exit. Reboot and load your new kernel!