How can i increase bandwidth from more isp?

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http://serverfault.com – I have some http server in Microsoft load balancing. Dns is served by hosting provider. I want to increase users bandwidth by adding another internet isp to my lan. How can i assure aivalability when a carrier is down? What i need? What kind of router? How can servers remain on line if static ip is provided by a specific carrier? (HowTos)