I have my CPU clocked at 1.1Ghz instead of the stock 1.533Ghz since my FSB is running at 200Mhz instead of 266Mhz. Is this type of underclocking conducive to heat reduction? If not, where could I change the actual voltage? (I have a Gigabyte GA-7VRXP Rev 1.1, and my BIOS doesn't let me change voltages).
Has anyone been able to run this particular CPU (or one of higher a rating) in a near fanless system?
There is a linear relationship between clock speed and power consumption, whereas the relationship between voltage and power consumption is quadratic. This is why undervolting is more effective than underclocking at reducing heat dissipation: the big gain is from undervolting, and the underclocking is mostly necessary so that the CPU is able to work at the reduced core voltage.
All thing being equal, underclocking the FSB from 133 MHz to 100 Mhz results in a 25% drop in heat generation, which in turn should result in a 25% decrease in the difference between core temperature and ambient temperature. The actual power consumption of an Athlon XP depends on what type of core it has. The newer Thoroughbred chips generate much less heat than Palominos, especially the "B" revisions.
As regards being able to run fanless, I doubt it. Even if you have a Thoroughbred "B", AMD's datasheets say power dissipation is 46.3 W at 1533 Mhz. At 1.1Ghz, it should be about 34.7W, which sounds too high for fanless operation.