The first Steam Machines prototype are very high-end PC hardware. Valve says they're all "off the shelf" parts and the only custom hardware in these prototypes (aside from the Steam controller) is the enclosure/case. Valve will also be opening up the CAD design for the enclosure should anyone want to build their own.
For the high-end prototypes, they'll all be powered by NVIDIA GPUs and use Intel CPUs. The GPUs will be a mix of NVIDIA GeForce GTX 780s, GTX 760s, GTX 660s, and TITANs. The CPUs will be some Intel Core i7 4770, Core i5 4570, and i3 CPUs. The RAM will be 16GB of DDR3-1600MHz memory and storage will be a 1TB HDD with 8GB SSD hybrid.
The Steam Machines prototype are very high-end and using a mixture of different hardware so they will be able to gauge what hardware works best and delivers the best performance for them. For the consumer versions, Valve will likely be backing low, mid, and high-end versions depending upon your price-point.
Overall, I am happy to see the use of Intel "Haswell" CPUs with NVIDIA GPUs. Haswell processors work great on Linux and deliver exceptional performance-per-Watt. Valve will be using the binary drivers with the NVIDIA GPUs, which work great and deliver the best performance and close feature parity to Windows under Linux.
More of the Steam Machines hardware prototype details can be found via SteamCommunity.com.