Route Policy With Linux

In one of our premises I have two firewalls; A Linux iptable and a Microsoft ISA Server.
I got two Internet connections, each connected to one firewall and the plan is to remove the ISA server and add the Internet connection (currently servicing ISA server users) to the Linux box which makes two internet connections on the same server. I got around 20 VLANs and I want to split Internet traffic between these two connections based on the source address. This can easily be done by iproute2 suite. I have not done this before but I am studying it and it seems easy and straightforward. I will post more on this later.