I've got 384Mb in a Wallstreet series 2 which has a max memory of 192Mb. Eventually I'll probably get 512. The only thing was that I had to pay more to get 66MHz RAM because the cheaper (PC100) stuff which should have worked according to lots of people, didn't.
The original limit was set because there were no low-profile 128MB modules for the bottom slot so they had a big 128 in the top and a low profile 64 in the bottom. I now have a low profile 256 in the bottom.
If you can find the right type of memory, my initial thought would be that you could get away with it. These things are not always so simple tho. If it is possible, somebody's going to have done it and no doubt, written about it online somewhere, so keep an eye out for articles/threads!