Payam:
The SiLabs company has some parts that have 128K bytes of FLASH. This flash is configurable so that it can be accessed at run time with either MOVC or MOVX instructions. In particular the MOVX method is how the FLASH gets written when the FLASH writing enable has been turned on.

These parts, like most other modern counterparts, are 3V parts in high density surface mount packages. As such your unlikely to find high performance in older style small pin count through hole DIP parts. If you want big FLASH XDATA with a 5V DIP style 8051 you are looking at a design with an external FLASH chip and its attendant address and data bus logic.

You may also want to consider an alternate modular design where in you use a removable flash module such as a SD Flash card wherein you get lots and lots of storage with a real simple serial interface.

Michael Karas