How many database licenses do we need for setting 2 servers (nodes) into a cluster

Performance will be very similar. GHz difference is not essential because e.g. E5-2690 has 3.8GHz Max Turbo Frequency vs 3.5GHz on E5-2643. You can find full comparision here:

http://ark.intel.com/compare/64596,64587

The big thing may be licensing. If you are using Standard Edition or Standard Edition One per CPU licensing they are licensed per socket. So in case you buy second E5-2643 you will also have to buy one more Oracle license. Of course that it not the case with the Enterprise Edition.

If licensing is also not an issue then you still have two decide between two choices:

• buying 2nd E5-2643 will be just cheaper now. Why not to save some money if everything else is the same?
• if you buy E5-2690 and after some time you will hit CPU bottleneck again you will have an option to buy 2nd E5-2690 and once again double the performance.

Also with such amount of memory you really really should use Huge Pages. Of course that means no more AMM and back to ASMM. But especially if you have many connections to Oracle the difference can be really big. You can read about configuring Huge Pages e.g. here:

http://www.oracle-base.com/articles/linux/configuring-huge-pages-for-oracle-on-linux-64.php

"Is MySQL suitable for distributing or cluster databases ?"

While you certainly can do manually sharding, the truth is that the most common MySQL cluster technologies are focused in high availability. NDB Cluster is based mainly on a memory database with lots of throughput, but not necessarily low-latency and large datasets. Galera does not support autosharding (only does read scaling).

"Is there any easy to use database which support clustering and distributing systems?" (I know MySQL have a cluster version but i don't know if it is appropriate in my project).

Other people may give you better advice about other databases, but beware:

• Many vendors that criticize the relational and transactional database model sometimes forget to tell you ideas like "eventually consistent" or the possibility of data loss in exchange for better performance (I look at you, NoSQL).
• Can you use a non-distributed database? There are datawarehousing/highly compressed solutions for MySQL (InfiniDB, TokuDB). Even InnoDB has a memcached interface (allowing you even 1M Querys Per Second), which may help in reducing latency for the SQL (although not memory requirements and disk space).

If you tell us more about how you are going to use your logs (statistics, historic data?), we may be able to tell you if a relational database is better for you, or you have to go for other paradigm like column-based databases, or map-reduce, etc. Even then, you should try any product for yourself.