I'm not really sure, perhaps if you posted your question a motherboard forum you might have better luck. Here is just one example of a motherboard forum:
On my Soyo motherboard I use the autodetect option in one place. In other places I had to manually set things because my BIOS was slightly too old to know the correct settings. I could have re-flashed the BIOS chip with a newer BIOS that would recognize it but I didn't.
Despite not being sure, I will make a stab at anwering anyway. Not knowing the correct answer for sure has never stopped me. So anway, I am assuming that the clock ratio is probably more or less the same thing as the clock multiplier. The clock multiplier determines the speed of the CPU. I believe that the multiplier probably refers to the speed of the front side bus times the the multiplier. What gets tricky is that I believe that there are two channels in the FSB each running at half of the rated speed for the FSB. If I am not mistaken, if the DRAM clock is set 166 MHz the fsb would be running at 333 MHz. So on a computer with a 333 MHz FSB should the multiplier be multiplied by 166 or 333? I am not sure which. My Athlon XP 2600+ uses a 333 MHz fsb. Your Athon XP 2400 probably uses a slower FSB like 266 MHz or something.
Another setting in the BIOS of some computers is the CPU to PCI divider. That has something to do with how fast the PCI slot runs. I believe that PCI slots are designed to run at 33 MHz.
Another confusing detail of AMD processors such as the AMD XP 2400 is that they do not actually run at 2400 MHz. What the name means it that they perform as well as a comparable Intel processor that runs at that speed. I actually have my AMD XP 2600+ running at 2074 MHz. From what I was told by one knowledgable person that is close to the speed that it should run (but is just a tad slow). Well at least my computer seems to work great and is still running over a year later. At the time I had Red Hat 7.3 installed and it came with a program that said my computer was running at 2074 GHz. I use Red Hat 9 now instead and it does not give that information. I also had a diagnostic program installed under Windows that said my computer was running at 2074 MHz and that the FSB was 333 MHz. You should probably find some program that will give that information to you.
Well, that was my lame attempt to explain something I only half understand. Isn't that what most of us do on this forum? Don't call me if you burn up your CPU or something. Until you are sure it would probably be better to underclock than overclock. One week ago I started a computer repair class. We have not yet learned much. By the end of the semmester I will know how I should have built and configured the computer that I have been using. You can take my comments for what they are worth. Building a computer from scratch and installing Linux and Windows on it was a good learning experience. It works great but I made a few minor mistakes along the way.