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Thank you for the post. The tech spec shows X520-T2 NIC is optional network adapter (recommended model) for Dell Precision T7600
You may check with Dell* support if X540-T2 has any compatibility issue with Dell* T7600 workstation, normally the system vendor is the one validate the compatibility of the network adapter added on their system.
Checking on the Dell* website http://www.dell.com/support/article/ph/en/phbiz1/SLN290764#Network, the system came with two onboard NIC, can you try disabling the onboard NIC to see if this will help?
If you want to set the PCI-E in the BIOS, please check with Dell* support.
Hope the above information could be of help.