For DIMM's, there is no theoretical difference between 128 1MB sticks and 1 128MB stick. However, you could theoretically have problems with certain sticks of different brands working together. This shouldn't ever pose a problem, and it's easy to fix.
By the way, if you're running win98 that extra 128MB does no good. I'm not sure about WinME, though I think it should be the same story.