You can up the core a little bit, but don't overdo it... The Northwoods should be kept below 1.7 volts as I recall or you risk "sudden Northwood death syndrome"
I'm surprised it wouldn't take the 155 speed. You may be running into a problem with overclocking your memory. You should set your memory multiplier to 1:1 while you see how far your chip will go, then figure out your max memory speed and timings.
Same with the other busses... lock the PCI in at 33 MHz. and the AGP at 66 MHz. You want to eliminate as many constraints as you can so you can figure out how far your processor will overclock.