First, 128bit and 256bit? That's confusing.
If you mean you have a 128MegaBYTE and a 256MegaBYTE SDRAM (or DDR SDRAM) DIMM, then yes it should run fine. If the mobo supports dual-channel memory you won't get dual channel (but you won't get dual channel with 1 stick anyways), but it should work fine.
If you really mean you have a 128bit chip and a 256bit chip, then no, they won't work because the socket on one of them won't fit.
If neither of the above is true, then I'm confused. :?