The first, "Opus" is basically a third-generation ("Falcon") board built to fit first generation ("Xenon") cases, primarily for the recycling of red-ringed systems, giving them cooler, more dependable guts (though no HDMI). The second, "Valhalla", is rumored to combine the 65nm CPU and 65nm GPU into one awesome super chip. It would only require one cooling system, meaning Valhalla would be quieter and (one would hope) more dependable. It would also be considerably cheaper.
Both chips (again, if Metcalf is on the money) are a ways off, with Opus coming in 2008 or 2009 and Valhalla following a year later. As excited as we are for the new builds (even though our eyes are getting sore from exchanged looks) we'll be happiest if this news means Microsoft won't euthanize the 360 as quickly as its predecessor.