it's best to google this, but many sources say that when your working set outgrows your RAM size the performance will drop significantly.
Sharding might be an interesting option, rather than adding more RAM..
Can MongoDB work when size of database larger then RAM?
What does it mean to fit "working set" into RAM for MongoDB?
You might also want to read-up on the 4square outage last year:
you said "swap is off" ... ? why? You should always have a sufficient swap space on a UNIX system! Swap-size = 1...2-times RAM size is a good idea. Using a fast partition is a good idea. Really bad things happen if your UNIX system runs out of RAM and doesn't have Swap .. processes just die inexplicably.. that is a bad very thing! especially in production. Disk is cheap! add a generous swap partition! :-)