Uh... I can't believe no one seems to know this but there is no such thing as DDR 133MHz (at least I have never in my life heard of it). The DDR you have runs at 133MHz, however DDR stands for Double Data Rate, which means that the data rate of 133 MHz is doubled to equal 266 MHz. If you say your memory is 133 MHz, make sure it is not regular SDRAM, instead of DDR SDRAM. The best way to check this is not to look at the memory you have, but to find out what is compatible with your motherboard by going to the its website.
But... if I were you I would just get 512 MB or 1 GB and dump the 256 MB because memory runs better when all the modules are exactly the same (mostly because of CAS speeds).