The A500 Serial device is only good for speeds up to 38400 BPS in my experience. BaudBandit.device is faster, up 56K max. I am currently connected using an A500 -> Null Modem through RAS on an NT Server machine running Proxy 2.0. I have also used Browsegate as well as other proxxy servers. I never did get my XP machine to share the net via ICS. Maybe someone else did... :-?
I use Miami and described my config at this
]old thread.
There is also this:
ipEther232.Telnet, but not sure how useful this would be on a stock A500.