First, forget the 8031 and EPROMs. If you want something 51-compatible, 
there are plenty of faster alternatives with Flash-ROM, e.g. by Atmel 
and Maxim. The easiest way to add an Ethernet interface to that is the 
ENC28J60. A few good IP stacks are available for free (uIP would 
probably be the best in this case).

But you will need a C compiler, so you might just as well start with a 
modern controller architecture, like something ARM-based. The AT91SAM7X* 
comes with built-in Ethernet, and you can get free example programs, 
some with RTOS, for many of the popular boards. If you want Linux, you 
need a bigger controller, but it doesn't make sense to use Linux just 
for Ethernet/IP.

If that's all too much and you just want a simple drop-in RS232 
replacement, take a look at the Lantronix Xport. It will work fine even 
with the real 80C31.

If you want to use 8051, then look on the Maxim DS80C400 Evaluation Kit:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4983

The DS80C400 contain a single cycle 8051-core whith 16MB address range, 
four data pointers and 16/32-Bit Math Accelerator.

The internal Ethernet controller supports Network Boot Over Ethernet 
Using DHCP and TFTP.


Peter