Sorry for being unclear. "[...] for accessing 16-bit RAM" was badly written and should have said "[...] for accessing RAM in a 16-bit address space" I was not discussion the width of the data bus, but the address range.

People did complain about the segmented architecture of the 8086/8088. But it has a huge set of addressing modes for a 16-bit address space, and it has quite good methods to use a 20-bit address space. The 8088 16-bit addressing modes do win by a large margin compared to the 16-bit data addressing of the 8051. The 8088 20-bit addressing modes do win by a large margin compared to a 8051 trying to access more than 64kB of RAM. The compilers for the 8086/8088 had perfect integration for 16-bit and 20-bit addressing. The 8051 needs manual banking or non-standard compiler extensions.

That should give an indication to people that a 8051 can be used with more than 64kB of RAM (and Dallas etc do sell processors with 128kB or more of onboard RAM) but that the OP should be quite clear exactly why he wants a 8051 processor with very much RAM instead of a processor originally designed with a 32-bit memory address space.

The OP has the right to get as good advice as the forum can give. But that advice should include the warnings, and recommendations to look at other processors. And it should include the recommendations that the OP comes back and gives a fuller description of the problem and why he need more than (probably way more than) 128kB RAM with a 8051. I am expecting this list to have the worlds leading experts on the 8051 architecture and that people knows both what the processor is good at, and what it isn't so good at.

I am assuming that the OP has already spent a bit of time looking for 8051 processors with much XDATA, and have found quite a number of them with 64kB and probably a couple with 128kB. The parametric search on Keils web site indicates that Mentor Graphics Co. has a chip with 1MB XDATA, but without downloading the datasheet, it is quite likely that this may be flash memory accessible within the XDATA address space. But anyway - I am assuming that the OP considers 64kB RAM little and wants more. Possibly quite a lot more.