Well MongoDB isn't limited to 32-bit systems at all, it works on both 32 and 64 bit but it's much better on 64 bit,
Second performance wise it's much much faster that any SQL-Driven db cause of the fact that it keeps indexes in memory (which consumes some memory) but it's very fast, also its insert and read rate are far better than mysql and much-less resource consumer.
One of the greatest benefits of Mongo is that it's structureless DB i.e. each record can have its own structure of fields, i.e. if you decided to add fields to record in the future you don't have to cascade all these changes to the old records (but you can).
Finally if you're thinking about apps that scale widely then forget all the relational DBs and do it in your code, because JOINS for example creates a temporary tables which is a db killer for large amount of consequent users. e.g. Google Big Tables, Facebook's Cassandra, Even Facebook uses MySQL as a key-store DB without relations or Transactions.