Postgres Plus Cloud Database on AWS for write heavy social game? I'm one of four founders of a big social gaming iPhone app in Sweden. We now suffer from severe database server issues so I'm turning to you guys to get some help.
First of all, let me get this out there: I'm not the tech guy in charge of the servers but I assist with research. 

We got about 8k querys/5 sec to our PostgreSQL DB and we're growing fast. The DB server is already behaving stressed out. It's a 8 core VPS with 16 gb ram. 

We're looking into the possibilities of using a Postgres Plus Cloud Database solution from EnterpriseDB running on AWS. (http://www.enterprisedb.com/cloud-database)

1) Anyone have experience in this matter?

2) We've also been looking into the options to use a dedicated server but we need the possibilities to scale the server up to 20-30 times. Is this relatively easy with EnterpriseDB's solution?

3) If i got it all right: EnterpriseDB offer a cluster solution which theoretically would solve our problems. Am I right?
For high performance systems, I think virtualization usually causes a lot more problems than it solves.  Save yourself the headache.  Build a decent db server inhouse.

It is hard to say what your current problem is, and virtualization makes troubleshooting much harder.  Is the load cpu bound?  Is it I/O bound?  Are you having concurrency issues?  Something else?

Now, if your 8k queries every 5 sec are not very complex queries, then the immediate question is why are you having performance problems.  It may just take some db tuning.  I have seen some indications that you should be able to get several times that on good hardware if things are well tuned.  Again virtualization is probably making things harder to fix rather than easier.

Finally, I would highly recommend looking into Postgres-XC.  It allows you to do sharding for write-extensibility while managing concurrency and consistency a layer above.   I would start tuning what you have and move from there though.