The 5500 and 6500 have only one drawback over their slower 5400 and 6400/6360 breathren - lower maximum RAM.
The 5500 and 6500 can only take 128MB of RAM, since they lack the 8MB on the motherboard of the 54/6400 (that give them a max of 136MB).
A cursory examination of the 5500/6500 motherboard shows four empty DRAM pads that ought to take the missing 8MB. All the hardware appears to be present that would allow a ceiling of 136MB _EXCEPT_ the DRAM modules.
Has anyone tried to solder in 4 DRAM chips from a scrap 54/6400? I've got a few spare boards and half a mind to do so, but I'm sorta busy with moving houses at the moment... If someone has some corroborating evidence, I might be made to spare some time and try it myself...
(No, I haven't bothered to download the dev notes just yet...)