NoSQL master master write performance

We are rewriting an application which is write heavy (wont go into exact numbers but +00's of writes per second currently).

The original application was using a postgresql back-end (now severely struggling) but we are wanting to move to NoSQL due to changing schemas etc.

To give an idea of our setup.

We have two application server sets. One in the UK the other in the US, but only one database which is in the UK.

We want to have a setup where by as the company expands we can create more application server sets in different locations each with an application server and datastore.

Because write speed is more important than read we need a solution which will offer us the ability to add extra datastore nodes in order to increase write performance with the data being replicated across other locations.

I would like to know what people suggest as a nosql datastore which will offer us the ability to have master:x:master replication and a high write rate.