Truth be told, there's no such thing as an ideal platform.
The ideal system configuration for you depends on the number and type of applications you typically run. Having said that, there are some general rules of thumb you can use as a starting point. For Win 98, you should consider a Pentium 133 or better and 16MB of RAM as the minimum acceptable configuration. Such a machine will let you run the OS and one or two applications comfortably, without a lot of disk swapping. Of course, the more applications you want to run at one time, the more memory you'll need.