If you have Windows95 with only 16-32 megs of memory, you will see a significant increase in performance if you go to at least 64 megs.
Prices have gone down dramatically in the past few years. Your computer won't be swapping to the hard drive nearly as much.
You also need to be aware of the memory requirements of the applications you are using. Office2000 requires a lot more memory than Office97 to get similar performance.
Also, the maximum useful memory for Windows95 is 64 megs. It doesn't do well with memory over that and may actually reduce your performance. Windows98 has removed that bottleneck. .
For Windows2000 I recommend at least 256 megs of memory. I tried it with only 128 megs and noticed quite a difference between Win98 and Windows2000. I currently have 384 megs and am quite pleased with the performance.