2006 Mac Pro connected with a fiber patch cord

I'm not sure why you asked in the iMac discussion area. I have asked the moderators to move your post to the Mac Pro discussion area.

Your Mac can do Gigabit Ethernet directly over 8-conductor Ethernet copper cables directly from either of the two Ethernet ports.

It seems like there should be a cheaper solution that just does the conversion.

Most PCIe cards MUST SPECIFY that they work in a Mac, and they especially need to work with 10.11 ElCapitan, or when you upgrade they will stop working.

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Have you considered this card?

http://www.sonnettech.com/product/presto10gbe.html

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That card will work in your Mac Pro at least up to and including OS X version 10.9.5 (Mavericks). They do not say if it will work with later versions i.e. Yosemite and El Capitan.

There have been quite a few similar products that have turned out not to be compatible with El Capitan but I would think there is a good chance it would work with Yosemite.

For the benefit of others, there are four typical reasons for wanting to fit an Ethernet card to a Mac Pro.

1. To add an additional network connection
2. To add a faster network connection, e.g. 10Gbps
3. To replace a faulty built-in connection
4. To support fibre for the purposes of a longer distance connection

The SonnetTech card listed by Grant is a 10Gbps card not a 1Gbps card and will therefore only work with a 10Gbps network switch connection. As it is possible to do 1Gbps Ethernet over copper cable for a distance of 100 metres it seems that no-one else bothers to make a 1Gbps card for Macs.

An alternative approach would be to get a cheap network switch which supports a 1Gbps fibre connection and locate it next to the Mac, then run standard twisted pair (copper) from the Mac to the switch at 1Gbps, and run a fibre 1Gbps link from the switch back to the rest of the network.