Well, it's kind of hard to tell, since we're combining our MongoDB nodes with other services too (Hadoop DFS datanodes too, to name a common example). We're running MongoDB on 12 different virtual dedicated servers, comparable to a dual core Intel Xeon with 4GB ram.

But I can tell you that you really shouldn't worry about MongoDB and performance, unless you want a really high load on a single server; I've heard it being able to do 20k write ops / sec on a single commodity server. From what I can see, our results kinda line up with that: MongoDB performance certainly isn't a problem.