8800GT 630-9368 HELP

Buying a failure-prone and barely adequate 8800GT at this late date is not recommended.

Here

About graphics-card compatibility between Mac Pro models - Apple Support

Apple says:

Graphics cards for Mac Pro (Early 2008)
NVIDIA GeForce 8800 GT (part number 630-9191 or 630-9897)

Mac Pro (Original) (included 2006 and 2007)

NVIDIA GeForce 8800 GT (part number 630-9492), offered as an upgrade kit.

Note: The NVIDIA GeForce 8800 GT card requires at least Mac OS X v10.5.2 with the Leopard Graphics Update 1.0 or the computer may not start up properly.

Googling

http://www.ebay.com/itm/Genuine-GeForce-8800GT-Apple-630-9368-512MB-PCI-e-Video- Card-Mac-Pro-3-1-/381132804879

shows it is for a 2008.

this search:

630-9368 /macintosh

will tell you all you need to know about that part number as it applies to Macintosh.

User Tip: Mac Pro silver tower (2006-2012) Replacement Graphics cards

that article has links to this article on everymac:

http://www.everymac.com/systems/apple/mac_pro/faq/mac-pro-default-graphics-cards -dvi-dual-link-mini-displayport.html

that article has a picture of the genuine Mac card, showing its fans and ports, BUT -- you should not buy ANY version of that card today! they fail too quickly.

What do you mean by different thickness? Both eBay links are the same link.

Some, usually less capable cards are single width by the better ones are double width since the need more room

And as stated here is the reason:

This model has a more efficient double slot fan/cooler that expels air outside the case. This allows the card and your computer to run cooler. The only reason the single slot cards are more expensive, is they are much harder to find. We've found this model to be much better and longer lasting and best of all - it's cheaper!