Last night we switched our default machine type from Amazon's c1.medium (2 core) instances, to x1.large.
The new machines are featuring 8 cores, 7 GB memory, better I/O performance and 64 bit architecture.
If you are performing assemblies that are heavy on encoding, especially those involving multiple output formats, you should see a significant boost in encoding speed.
Unless something comes up, our plan is to only use 8 core machines in production going forward. And who knows, the Cluster Compute Instances (cc1.4xlarge) are only one step away now : ).
--fg