SATA drives are backwards compatible so the SATA 3 (6Gb/s) Samsung 840 Pro will work on the SATA 2 (3Gb/s) ASUS P5B motherboard.
SATA 2 (3Gb/s) speeds are from 151MB/s to 300MB/s.
Expect your Samsung 840 Pro to Read/Write from 275MB/s to 295MB/s.
Thank you very much for your support. At least i hope my mainboard has AHCI support and can get fast response speeds from my SSD.