133 FSB. Had to bump it to 1.7V, but the P4S undervolts the CPU as far as all the monitoring software is concerned. The voltage usually floats around 1.65V. Your overclock may be hitting the ceiling of your CPU. I haven't tried any higher since I'm perfectly happy with 2.4G and this leaves all my buses within spec. I have crucial ram so I'm sure the ram can go higher, but that'll throw off my AGP and PCI buses. Also from reviews I've read, the SIS chipset performs slower with asyncronous bus.
Your 120 FSB may be throwing off you other buses. Make sure you are using the right dividers.