There are 2 software-related issues involved. One is the operating system you are using. Win9X doesn't manage memory all that well so in the broadest sense, you wouldn't see that big a performance difference. On the other hand, if you are using Win2K you should see some difference.
Of greater importance is your applications. In general apps like Word and Excel, you're not going to see a real difference between 128MB and 256MB of RAM. The same goes for most games. However, fire up a program like PhotoShop and you will most definitely see a difference.
Finally, it probably goes without saying that if you really want to increase your PC performance, lose the K62 and get a Duron or T-Bird.