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Leo's suggestions are good, although the 860 might be only be "just enough" even in dual router implementation. 880 should be fine in dual but perhaps a little tight for single router implementation. (NB: BTW, Leo's suggestion would likely work just fine, personally I like a bit more reserve processing capacity.) What bandwidth growth, if any, do you anticipate?
Since you did mention redundancy, Leo's suggestion for two routers is excellent. However, if you don't already have it, also provide some LAN redundancy. For example, you might have each of your WAN routers connect to two different switches or to different stacked switches.
