What hardware makes a good MongoDB Server ? Where to get it?

Initially, you'll want to beef up on the RAM. The RAM you'll need is dependent on the amount of data you're storing, number of collections, indexes on those collections, data access patterns, etc. Lots of factors.

The most important thing is to have enough RAM to keep your indexes in RAM. Otherwise your performance will suffer dramatically as your server(s) will page constantly while Mongo moves memory mapped files in and out of RAM. In spite of all of this, we haven't seen write speed affected but everything else is. Processing writes off the queue, flushing, dumps, etc all take a dramatic hit once your indexes no longer fit in RAM.

So there is no real short answer. Basically, be smart about your indexes. Only use what you need. Keep collections small if you can (ie break out into multiple where you can.) Capped collections are also interesting to look into.

It is very important to use a 64 bit machine not 32 bit. http://blog.mongodb.org/post/137788967/32-bit-limitations

With MongoDB what you want is RAM. And then some more RAM. Buying RAM can't hurt.