I'm evaluating a number of options to an rdbms for some of our facebook games.  A small, yet important number of queries require transactions.  For example we have auctions, and buying/purchasing of items with limited quantities.  

Are there knobs you can tweak to enforce consistency on a per query basis?  For example, I want to do a write that should not be visible until it's been written to all nodes in the cluster, is that possible?  What kind of general time frames are we talking about to write data to 3 nodes?

So basically what we want is high availability for 99.99% of queries, but high consistency for the remainder, across the same data.  Is this currently possible?