ok, first you may want to make sure you get low density ( double sided ) RAM - chips on both sides of the memory stick. I believe this is what you have?

Next, get a brand name. Getting no-name RAM will cause memory issues. Had a friend who did the same and his computer would give him problems.

Try to get these brands:
Micron or Kingston

A few other very good brands that should work well too:
Corsair, Crucial, Mushkin, Infineon

I realize finding PC133 RAM will be hard but going for No-name ( low quality ) RAM is always a bad idea.