Is there any way to upgrade the graphic card of a Mac Pro to run the HTC VIVE ?

The only upgrades would be to replace the D300's with D500's or D700's. You can get them from eBay or other places. Apple does not sell them.

and the vive will absolutely work as advertised? I have the 6 core with the 500's and I've been searching on the web for somebody that uses that setup with a vive...(can't afford a new pc either as I spent it all on the mac)

Vive web site says a PC running Windows is required, and does not mention any of the D300, D500, D700 GPUs directly.

I have not tried this myself but based on similar queries and answers regarding trying to use a Oculus Rift on a Mac while for example the FirePro D700 has sufficient power to support the Oculus Rift it is not recognised and accepted by Oculus even when running Windows via Boot Camp.

The Apple cards in the Mac Pro 2013 are not standard cards and therefore get rejected by the Oculus driver.

I would therefore not be surprised with a similar situation regarding the HTC Vive.

Note: Steam have a VR performance test tool however I suspect it only checks to see if your computer has enough performance and does not prove the software will accept your configuration. See - http://store.steampowered.com/app/323910/

There are rumours that next year Oculus may start supporting Macs and OS X officially but I would also expect this at least in some cases may also be in anticipation of new Mac models.

So, if you run the above Steam tool via Boot Camp and Windows it will at least show you if your Mac is fast enough.

Ironically, if you had an older classic Mac you could fit a genuine Nvidia GTX-970 which is approved for HTC Vive. You would still need to use Windows and Boot Camp.

For VR realtime Experience with HTC Vive you need powerfull multiGPU setup. unfortunately, Apple does not offer that powerful VR valid graphics-hardware - you have to improvise.

THis is is how it works:

when using the current macpro: you either need to buy an external GPU box, connected via thunderbolt with at least 2 NVIDIA Titan X (Maxwell) or 980i installed - and install the latest NVIDIA webdriver for Mac.

when owning several 5,1 mac pros - put these cards into the internal PCI slots (with a 6to8pin power connector/adaptor. just google for "980i in Mac Pro") And install latest NVIDIA webdriver for Mac.

that works fine on OS X 10.11. But not yet on sierra, due to lack of driver support, until now.

Other Alternative (sadly): buy a windows PC containing 2 to 4 NVIDIA titan x (Pascal). This is currently the most powerful option for serious VR combined with HTC Vive.

and you need to bootcamp 🙂 for native support of Vive - sorry, forgot to add...