Okay, that’s not your usual power supply, and thus why I was getting a bit confused. This document puts it in perspective: ftp://ftp.dell.com/Manuals/all-products/esuprt_desktop/esuprt_dell_precision_workstation/precision-t5600_Owner’s%20Manual_en-us.pdf
Page 27 shows the board you mention, and yes, that would presumably have the correct pinout. Also, this document mentions that the single 8-pin PCI-E connector might be enough to power the C2070:
http://www.nvidia.com/docs/IO/43395/Tesla_C2050_2070_IG_September2010.pdf (see page 7)
That being said, I can’t find any pre-made 8-pin to 8-pin male PCI-E cables. It would seem like this seller had some at some point but raised their price to a ridiculous amount rather than removing the listing: http://www.ebay.com/itm/380488749075
You could always make your own cable – you can purchase the connectors and terminals from digikey or mouser and get the right gaugue wire and crimp them yourself, but that gets into being a pain.
Ideally, you’d want to connect both. I’m not sure if you have any free molex or sata power connectors in that server, but if you do, you could get adapters like this to populate the 6-pin PCI-E connector on the C2070:
For that matter, similar adapters to the ones listed above exist with an 8-pin connector. That being said, not sure if that would be a wise recommendation… depends if the power supply would be able to push that much power over the SATA/molex connectors.
Edit 1: Aha! Here is what you are after ;)
Edit 2: Actually that’s an 8-pin to a 6-pin cable, now I understand what you’re saying… you mean that this is the cable you have currently, correct? If so, you need something like this: http://www.newegg.com/Product/Product.aspx?Item=9SIA30R1SX5209
or even better:
So… basically… if you do not have the VGA1 Dell P/N 0MG89 cable I listed next to ‘Edit 1’, get that, and the last link above (the splitter cable from frozencpu)