Your Motherboard, your two Processors and your three Memory Modules are supposed to run at 266 MHz Bus (Everything further than that is overclocking all of these components!
, so it can be anything of these what is holding you back. However, most Athlons XP (Athlons MP included because they are Athlons XP with a bridge enabled for SMP support) with a good Motherboard and Memory Modules reach usually 400 MHz Bus with no real problems, so I suggest you discarding the Processors. Those Motherboards are intended for Workstation, that is why they aren't easy to overclock (Neither mean to be used for so), so it can be a possible reason, but your Memory Modules are suspicious too. I suggest you trying to overclock the FSB with only one of the Memory Modules at the same time, maybe one of them has reached it limits. If you want to discard Memory Modules, you will need a 333 MHz Bus or 400 MHz Bus Memory Module in order to be guaranteed that isn't that what reached it limits, and if you still having the same problems, then its possibily the Motherboard.