Upping the FSB does overclock the RAM -- and it also proportionately overclocks your CPU, AGP, PCI, etc.
Some mobos, as mentioned, allow an asyncronous 100fsb with 133mhz RAM. In addition to that, some BIOS' have very agressive timings that effectively overclock your RAM without increasing the fsb. For example, the default setting in most BIOS is to have the ram timing set 'by SPD'. Often, SPD sets your RAM at cas3. Instead, if you choose to manually set your RAM timings to cas2 then you are squeezing more performance from your RAM. My board (ASUS A7V) also allows the RAM to be set at 7ns and 143mhz. Additionally, enabling '4-way interleaving' and byte merge are agressive timings that will squeeze more performance from your RAM.
Of course, as with any overclocking, any of these settings can make your system unstable. My advice is to try enabling these timings one at a time and then testing your system for a few days before trying the next thing.
Personally, with a 1.4ghz Athlon (200mhz) and generic PC-133 sdram I;ve been able to get my fsb up to 110 so my RAM is running at 147mhz. Also, I have it set to Cas2 with interleave enabled and byte merge enabled.