Genuine Apple Ati Radeon Hd 2600 Xt 256mb Graphics

I will be purchasing a Mac Pro soon. As you all know it comes with the ATI Radeon HD 2600 XT w/256mb. Being that I am a PC user I have in my possession a Dell gift card. Now, at the Dell online store I am able to purchase another ATI Radeon HD 2600 XT for less than half of adding a second card at the time of purchase, but this one comes with 512mb. I'm not 100% sure but by comparing images at Mac and ATI, I think the two cards are manufactured by the same company which is VisionTek. Does any body know if these two cards will work together or even if the 512 will work at all?I have researched ATI and they don't even aknowledge the 2600 as a Mac compatable card. I have tried calling Mac, they said come here.