I bought a NAS from Conceptronic CH3MNAS and built in two Western Digital 1,5TB Green Drives.
I only get a write speed of 6mb/s in LAN
The configuration of the drives is as follows: - Raid 0 - EXT2
Is that a normal speed?
Assuming you are on a 100Mb/s network and that you actually mean that you are getting 6MB/s this might be right.
6MB/s = 48Mb/s. Actual TCP/IP throughput on a 100Mb/s network is around 80Mb/s.
The 32 or so Mb/s that you are missing in performance can be for a number of reasons. Old NIC drivers, bad/old network cables, cheap networking equipment, etc.
In support of Zoredache, those Green drives are very, very slow and are NOT effective in a RAID environment. I am actually using 2 1TB green drives in a NAS RAID1 and the performance is horrible. I didn't do my research before setting this up and luckily for me the usage of this device as secondary backup means the performance hit doesn't bother me (and the RAID1 means I'm not too concerned with a failure). I would recommend you switch to the WD Blue series for reliable NAS usage. Do not use the WD Black for RAID as they will drop out of configuration when they power down.
If you have the money, the RE line of Western Digital drives is your best bet as they are designed for RAID configurations. They are definitely more expensive.