> 1. How much memory is required to run pppd (dial to ISP) in Linux?

I am not exactly sure here. I believe this will depend on how good
your linux build is and what machine you are putting it on.

> 2. Can anyone introduce embedded Linux board that run pppd and allow
> to code RS-232 and socket programming in C language ?
> Minumum 2 serial ports are required.

I am currently using a SBC which is basically a embedded PC on
steriods, or a very malnurished PC. The total available memory is
16M. 32M is available. I am running a optimized 2.4 kernel and
utilizing pppd to maintain a internet connection via cellular modem.
The SBC i am using has 5 serial ports, 2 usb ports, and the ability to
run LCD and touchscreens. The company we got the SBC from has other
smaller versions available. www.arcom.com

It is not exactly embedded, but the reduced development time edged us
to it.

Robert