OK.
I have upgraded to a Quad Core CPU (intel Q8300) with 4 GB of ram and a mobo with intel chipset (Don´t remember wich one).

The performance is even worst!!!

I only have a maximun of 15.000 Kbps on each Nic and a total usage of 28647 kbps (according the esxi graphics)

I give up with this. Tomorrow i think that i will go back to Openfiler.

Thank you all!!!