The issue with large SIMMs (over 1 MB) in Mac II and Mac IIx is the video cards. 4 Mb chips added a new test mode, and it happens that the TFB video card would cause signals on the buss which would lock RAM into this test mode. Therefore, 4 MB and larger SIMMs for Mac IIs need a PAL or other device to lock out the problematic RAS / CAS sequence. You DO need a PMMU memory controller to use more than 8 MB, and you need either 32-bit clean ROMs from the FDHD kit or MODE 32. The 32-bit ROMs are compatible with both IWM and SWIM disk controllers. IMHO, unless you've got some of the special RAM on hand, it's not worth the effort and cost to expand a II or IIx beyond 8 MB. The software which ran on those Macs will run quite happily in 8 or less.

It's possible that a later model SuperMac or Radius video card would be compatible with large RAM, but I never heard of anyone testing such a setup. If you actually want to USE a NuBuss 68K Mac with lots of slots and beaucoup RAM, look for a Quadra 950. Those old beasties can be found here and there for cheap, and they'll accept up to 1/4 GB of RAM.