From what I've read I would guesstimate that if you switched to 256 MB of PC133 CL2 SDRAM @ 133 MHz CL2 (like Mushkin Rev. 1.5 or 2) from what you have now, the benefit in overall performance would be like increasing your CPU speed maybe 10% max if you are running your RAM @ 100 MHz CL3 now, or 5% max if you are running your RAM at 133 MHz CL3. And I think that's a max overall benefit. Only you can judge whether that is worth it.
Depending on what you do with your PC, as Noriaki points out, 256 MB s/be enough with Win 98, but maybe more with Win 2K. Personally I think the high-end RAM is most useful if you're overclocking to an extreme where the memory bus speed is very high.
Now if you need more RAM and don't have a DIMM slot free, well ....