If just for serial application and some I/O for other stuff, then I would recommend cheap and easily available :

AT89C1051 - 1K code
AT89C2051 - 2K code
AT89C4051 - 4K code

the above mentioned chips has an UART built in and the footprint is only 20 pins. Difference between them is only the code size and they can take a crystal of 24 MHz (maximum).

For more info, go to http://www.atmel.com

And you will also need a RS232 transciever like MAX232 where you can look for more information at
http://www.maxim-ic.com

Hope that helps !

Best Regards
Andy