i believe that the 845PE has a quad-pumped 133 MHz FSB (533 MHz).
pc2100 is 266 MHz DDR. the 266 MHz DDR is just 133 MHz x 2 for the ram. if you are overclocking your current system, you are likely to run into stability problems since your ram is only rated at 266 MHz DDR. when you increase your FSB, your ram is running out of specification, which can cause a lot of issues.
get faster ram if you are going to overclock your current system. if you have a P4 3.0 C, you may want to get a newer board since you are limiting the fsb of your cpu.
vehementi, the 845PE does support a 800 MHz FSB CPU (200 MHz x 4) but it will limit it to a 533 MHz FSB CPU (133 MHz x 4).