Very doable, but you need to evaluate your options carefully. You will discover that the better your ISP diversity (improving availability, if done right), the less effective and more difficult your load balancing.
My general rule of thumb is that for a reasonable level of effort, you can get perfect load balancing using multiple links to a single router at a single ISP, reasonable load balancing using diverse links to multiple POPs of a single ISP, and lucky to be within a two to one ratio using links to two independent ISPs.
Of course the other side of the coin is the size of the failure required to bring down your internet connection. But use care in your design so that the diversity you're paying for is maintained all the way to your computer room, if not to your users. It's amazing how the construction guys always pick the most critical links to cut :-)
The is a brief white paper outlining your ISP connectivity options on my web site, and several example configurations are explained in my book. Plus there are many examples here on CCO.
Good luck and have fun!
Vincent C Jones
www.networkingunlimited.com
