Well the part number for the one you linked is Dell Part #: 320-4570.
When you configure a single 256 GTX in a XPS 600, the part number is:
256MB Nvidia GeForce 7800 GTX NV7800 [320-4374]
When you configure a dual 256 GTX system, the part number is:
Dual 256MB Nvidia GeForce 7800 GTX with SLI Technology NV7800D [320-4373]
So the part number is definitely different then the ones Dell uses in their XPS 600. And it does look like a beefier card then the normal 256 GTX.
Edit: also found out another number on that card: UNSPSC: 4320140100 if anyone knows what that means...eh nevermind, that's listed under every video card.