2 ISP Problem

I have two ISP`S and i want that the half of my customers surf on the first ISP and the other on the second ISP.
I have 3 ethernet cards.

192.168.2.2 for ISP1
192.168.50.1 for ISP2
192.168.178 for the Access-point

How can I manage it.

Please help me

http://wiki.mikrotik.com/wiki/Routing

http://wiki.mikrotik.com/wiki/Load_Balancing