Setting up Mongo with clustering
You can do it either way. 

There isn't much benefit to starting all of the overhead for sharding now, unless you simply want to try it out to see how it works. It is important to pick the correct shard key, so you would want to make sure your schema and insert/update usage is firm enough to know what to pick. 

Kristina Chodorow has a great blog entry about how to pick a shard key: http://www.snailinaturtleneck.com/blog/2011/01/04/how-to-choose-a-shard-key-the-card-game/

For general information about how sharding works:
http://www.snailinaturtleneck.com/blog/2010/03/30/sharding-with-the-fishes/

For some instructions on how to set up clustering on a single machine, see here:
http://www.mongodb.org/display/DOCS/A+Sample+Configuration+Session
and here:
http://www.snailinaturtleneck.com/blog/2010/08/30/return-of-the-mongo-mailbag/