well your board isn't made by the best of comapnies. overclockers and enthusiasts wouldn't even consider ECS. but we should be able to get a little bit out of it.
first off, make sure your memory's running synchronous to your fsb. in other words, 266mhz, equivalent to pc2100. synchronous is almost always better anyway.
next, disable some options in the bios. namely advanved pci and agp functions like fast writes, agp master 1 ws write, agp master ws read, and pci 1/2 master 0 ws write. these were listed in your manual so you should have them.
then while in your bios, turn off all ports and devices you don't use. includes, game port, parallel port, serial ports, firewire, modem, sound, floppy controller, etc. a lot of these devices run off the pci bus so any one of them could be limiting it.
try that for now and see if you can get any further.