If you're up to W98SE then you're out of the problem areas with RAM. So, in general, the more the better.
However, don't forget to create, or let windows create, a swapfile or pagefile that is at least as big as your RAM. The W2K default is 1.5 times the RAM you have fitted and to alter this you have to go deep down into the management bilges of the OS.
Have a look in your root directory for the size of the file. If you want to increase it look at the Help File and it will tell you how to do it.