PC133 memory should work ok with your system though will only run at FSB speed of your moby, probably 100 in your case. My memory is 128mb pc100 (original) and 256mb pc133 - - - only bought the pc133 as couldn't get the pc100 at that time, maybe 2 years ago.
Would get this new studd checked out - - -you should be able to use a stick of 256.
Someone told me, when I wopped my extra stuff in there, that the biggest stick should be in slot 1 - -so I took 128mb out of 1, stuck the 256 in that slot then 128mb into slot 2. I have since read somewhere that this doesn't make any difference. Just my 2 penneth.