a)If I replace the 89C66x on my board by a 89C51RD2, will FlashMagic still able to download my code ?
b)Is there any other difference ?
a) yes If It is P89C51RD2, but be aware that there is two RX2 versions, Rx2 and Rx2H. Initially there was only the Rx2H so some older litterature stating Rx2 is really about the Rx2H. FlashMagic will program both.

b) some SFRs and interrupt vectors have different addresses, just go through the list.

Erik