We've generally found that 1GB of RAM per 10GB of data is best for common Mongo setups. SSDs make a huge impact on the to-disk performance, which does help with

We've generally found that 1GB of RAM per 10GB of data is best for common Mongo setups. SSDs make a huge impact on the to-disk performance, which does help with the working set. SSDs also help with writes and can help mitigate the infamous global write lock problems in Mongo.