Dig out your motherboard manual and try a setting of 5x1??MHz, the higher the better. Keep in mind that you have to have ram that can take the higher speeds to do this, but most good PC100 ram (and any PC133 stick) can take <120MHz.
Since the level2 cache on your system is running at the same speed as the front side bus a higher bus speed is desirable. You will not notice much of an improvement if you just increase the multiplier. If you are lucky you might even get your system stable at 5x120 (600MHz). 4.5x120(540MHz) would also be a good speed.
As long as your temperatures are below 50c you are in the clear. Don't throw away tons of money on an old system like that just to get it to perform marginally better.
If the cpu refuses to run at a certain speed, try giving it a little more juice. As far as I recall the default voltage on your cpu is 2.2v, so giving it up to 2.4 is fine, watch your temps as you increase the voltage though. Don't worry about killing your cpu with the higher voltage itself, I've seen K6-2's take some insane abuse and still work.