9x can't really utilize more than 128MB of ram.
Yes it can. The 9x kernal has issues with *large* amounts of ram, however it can utilize more than 128. The biggest problem with 9x was that it wasn't designed to run with large amounts of ram, but it can and will. Generally you need to do some vcache tuning. Cacheman is a good program, and I've used it on some low memory 9x systems. But anytime I was over 128 megs, I really didn't need it.
On the other hand, the NT based OS's, such as NT 4.0, 2000, and XP, do not require any type of 'memory management' software. In all reality it will most likely be detrimintal to system performance. The NT kernal does a wonderful job of managing memory, and I'd prefer to use that over any 3rd party hack.
In essence, sure, some memory management software works fine on 9x, but I'd avoid the 'ram doubler' programs or anything that promises something along those lines. But I prefer to avoid them for the NT kernel.