what exactly do you want to load-balance?
I assume both networks are using their own nics in the pc and then a third goes to the WAN connection(T1, del, cable etc)
YOu load-balance EACH network by using two wan conncections, or two nics per network
To my knowledge, the best nics for this would either be the Intel Server adapter or a 3com solution. Check the websites.
You could use windows' load-balancing(It is in there somewhere-search for it) or quaity of service if network really doesn't need the bandwidth(old grannies checking e-maail on one side, teens downloading massive amounts of pron on the other)
good luck 😎
EDITED FOR CONTENT UNBECOMING SOMEONE OF MY STATURE😉