To give an advice which is best of all the quotations, it would help to know the details of all, not just one.
I'm wondering about that 8 TB hard disk. It might be faster to spread the load between 2 times 4 TB or even 4 * 2 TB. For a hard disk, SCSI might be better than SATA. And it's certainly much faster (and maybe worth the extra costs, although that's dependent on the business case) to have SSD in stead of HDD for a heavily used database.
And I've never seen a database server needing SLI.
One remark: a database containing 6 TB of pure data needs much more for indexes and other overhead. Did you take that into account?
Finally, the performance of a database heavily depends on the settings. It might be best to hire an experienced DBA to set it up for you. A company with 6 TB of data surely has the money for that. And certainly, such a professional could advice about the hardware also, given your expected work load.
I am looking forward in creating an oracle server with database approx 6 TB. I am pretty confused with all the quotations I obtained from different vendor. What would be the best system configuration to have for my database server. One configuration which I was thinking is
Intel Xeon E5-2640 v3
64 GB (16 GB x 4) DDR 4
8 TB (WD Re WD4000FYYZ)
GIGABYTE GA-X99-Phoenix SLI
Cooler Master HAF 912 Combat
Cooler Master RS750-AMAAG1-UK
Please help me choose the best configuration for my server which would be cost efficient and performance based

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