Gotta leave for the baseball field in two minutes so this will be short.
Memory in the Q605 is 32 bits wide. If there are only eight chips, then they are 4 bits wide each or X4.
(1M X 4) X 8 chips yields 4 MB.
Memory of the requisite type comes as large as 16M X 4, but the chips are extra large. Look up the D4893 SIMMs on Ebay for a photo. Those are the largest capacity memory chips you can use. With those you can get 64MB into a 32 bit wide bank. But the connections would probably be a nightmare.
the 4M X 4 chips are about the same size, and if the pins match up and just tack on a couple at the end, then this would be much easier to connect. Just make a connection for the additional address lines from the SIMM socket. The address lines should be common and just the RAS and CAS lines unique between the sockets, I think. I'd want to test that with a DMM though to be sure. If the address lines aren't shared, you'll have to hunt down the extra lines on the controller chip, but it doesn't make any sense for them to duplicate address lines when they could share them.
4M X 4 chips will get you 16MB in a bank.
Rob, is there a software reason why the on-board RAM in the Q605 must be treated as single-banked? If there are four RAS lines to the on-board memory, the RAS signals might be handled in pairs by the controller, allowing the installation of two banks of memory by correct pairing of the RAS lines.
That's how the SIMM socket handles a 128MB or a 32MB SIMM. It has four RAS lines, but they're divided into two pairs and in a double banked SIMM, one bank uses one pair of RAS lines and the other bank uses the other pair. On a single banked SIMM I think either all four or just one pair are driven.