You can try Triscend TE520 with his 40kx8 dedicated system RAM (XDATA RAM) but it's not cheap at all.
My favorite is Philips P89C668 (with 8k RAM and 64k flash inside), as excelent quality / price ratio. h**p://www.semiconductors.philips.com/acrobat/datasheets/P89C660_662_664_668_4.pdf
For a full overview go to h**p://www.keil.com/dd/parm_search.asp
and select the parameters.
You could finf what you need from major makers for MCS-51
MCU. However, IDATA area for any MCS-51 is 256 bytes
max. Generally,. the extra SRAM will be accessed by
MOVX @R0 or
MOVX @R1 or
MOVX @DPTR