Currently each server instance loads around 650MB of memory usage, and after 24 hours balloons to 720MB (However, after patch 1005 I haven't seen it grow much more than this, but test time was limited. Prior to 1005 I did see each instance grow to nearly 1GB each after a 48hr period).
On a 4 core Intel Xeon E5-2690 3.00GHZ processor I would see a peak load of around 10% during a wave 10, 6 player long match. Average load was around 5-6% per server instance.
All in all, the server requirements are pretty light.
FWIW, stats above are typical on both 2008 R2 and 2012 R2