Unless your motherboard has an HX chipset, motherboards designed for the Pentium/Pentium MMX can only cache a maximum of 64MB of RAM. Because of Microsoft Window's "top down" memory structure, all of the important stuff will be loaded at the top of memory, outside the cached area, resulting in possibly degraded performance.
Whether or not you will see any advantage in going to 128MB of RAM depends on your usage patterns. If you are using Win95, and use the computer for only light multitasking, like word processing and surfing the net, chances are you will see better performance with only 64MB of RAM.
If you are a heavy user with Win98, Photoshop and many applications open at once, you will probably be better off with 128MB of RAM. In such cases, even though the important stuff is loaded in the uncached area of memory, the fact is that there will still be headroom for the program to be loaded in RAM, rather than needing to be accessed from the pagefile on the hard disk, which is much slower. Win98 is much more of a RAM hog than Win95.