Thomas, thanks for the input, yes I saw that URL and it wasn't really clear.
So where RME 3.4 reports
RAM Size(MB) 6.00
and "sho ver" reports
cisco 1603 (68360) processor (revision C) with 6144K/2048K bytes of memory.
8192K bytes of DRAM onboard
The router has 8MB of DRAM, calculated by adding the on board 2MB and the 6MB SIMM? I guess the bit that surprised me was the existence of a 6MB SIMM. I thought it was a case 1,2,4,8,16 etc?
So to upgrade a 1600, take out the old SIMM (6MB in this case) and put a new one in?