What is the acceptable ratio of Polling engines (w/ NTA) to NTA database server?

I have three Orion environments that were upgraded to NTA 4.0, and they need to be fitted with new VMs to serve as NTA database servers.

Since we have multiple polling engines on each environment; what I would like to do is share a database server with two or three polling engines.

Assuming a relatively small amount of NetFlow/J-Flow traffic per polling engine (likely less than 1000 flow/sec); what should be the correct ratio of polling engines to database?

Here are my proposed specs for each NTA database server:

4 CPU

16GB RAM

600GB D drive for database

Win2008 R2