Your benchmark performance is poor (way better than any HDD) due to the NVIDIA SATA chipset, and it is SATA II, not SATA III. Frankly, your results are above average for a NVIDIA SATA chipset. Your 840 Pro is not at fault whatsoever, as said above, it's all about the mother board and NVIDIA SATA chipset, sorry to say. It all makes perfect sense to SSD enthusiasts.
What does the main screen (Disk Drive) of the Samsung Magician software show about AHCI Mode and SATA Interface? That will also provide information regarding the performance potential of your SSD when used in your mother board.
As mentioned above the, obsolete and unsupported NVIDIA chipsets are causing grief for SSD users. Your board's download page actually has a NVIDIA AHCI/RAID driver, for XP and Vista only, but we have no idea what OS version you are using.
Does the BIOS have a SATA AHCI mode setting? It is supposed to have RAID mode, but I'm hoping for your sake it has an AHCI mode setting.
We might be able to get performance a bit better, but then you'll hit the SATA II bottleneck. How does your PC perform with that SSD as the OS drive?