Currently running Windows XP Internet Connection Sharing this computer for 3 computers at home.

Cable internet plugs into ethernet card #1 on my computer
Cable from eternet card #2 goes from my computer to hub
Other computers connect to hub

Is there any way to limit the upload/download rates of the other machines through my computer?

try WinProxy

i'm not shure this is what you want. I've use this for many years to share a internet connection on windows and i remember it was doing very good it's job.
see http://www.winproxy.net/