The trick is !EA, this pin determines whether code reads are internal or external to the chip.

If you need more memory (RAM and/or flash)have a look at the philips P89C51Rx2 (up to 64k flash, up to 768 bytes of extra RAM) or the P89C668 (64k flash, 8k bytes of extra RAM)

I echo the sentiment above, why not use flash.

with the devices mentioned above you get IAP (in application programming) as well.

Have fun,

Erik