Try upping the FSB (From 133MHz) in small increments first (you usually get better performance from increasing that, rather than the clock multiplier, as memory bandwidth is the bottleneck - a 133x10 1.33Ghz system is faster than a 100x13 1.3Ghz ). Don't go too far, as your memory may not keep up. 140x10 would be 1.4Ghz. Watch the CPU temp though, don't want to burn that nice new Athlon. If it won't post, back off the FSB.
Of course, you don't really NEED to OC a 1.33Ghz system, do you?