I tend to agree with that view. The 8051 is good when it comes to manipulating bits and bytes but was never strong at maths. It simply is not a number cruncher and the 8-bit data width is kind of limiting when it comes to multi-byte processes which you will require to perform when doing a project like calculator.

As a general suggestion, have you had a look at the ARM processors ? I somehow feel that an ARM processor MIGHT fit in this slot well more than a 8051 or PIC ? ( Just a gut feeling based on the wider data width of the ARM architecture- maybe I am wrong..)

Raghu