Now all we need is to find out that the Quadra 800 or 840av can take 128mb simms, and can address all 512mb of ram that would result. "yaay!" would ensue.
Won't work. In fact in the 840AV SIMMs larger than 32MB are seen as 1/4 their rated capacity. In the 800/650/610 they are seen as half their rated capacity, so it is possible that installing 128 MB SIMMs might get you 64 MB of capacity per slot.
I did some extensive experiments a while back and posted them either here or on Applefritter. A search here doesn't turn it up, so I must have posted on Applefritter.
The 7100 and 8100 behave like the 840AV and 660AV in that they treat larger SIMMs as 1/4 capacity. The 7100 and 8100 have an extra logic chip between the memory controller and the SIMM slots, which screws up the support for larger SIMMs seen in the 6100.