I was in a situaation similar to yours, with 64 MB of PC100 that'd only run at 112 3-3-3 with a 500 MHz P3 at 560 on a good BX board, limited by the RAM and by the CPU both. First I got 128 MB of Rev. 1.5 and and added it to the PC100 - ran that combo at 112 3-3-3 too, of course. Then, with prices down I got another 128 of the 1.5 and deep-sixed the PC100. Ran the 256 1.5 at 118 2-2-2 with a new 850 P3 at 1000, limited by my CPU now and not the RAM. Personally, I don't need any more RAM than that.
Under Win 98 you probably have enough RAM at 192. With Win 2K you may need more RAM. It all depends on what you do with your PC.
I wouldn't swap out the PC100 just to get from CL3 to CL2, but if you are overclocking it is another story, since the RAM may be the limiting factor. Also consider whether your board has an asychronous memory bus.