i have a 7800gt 256mb card and was wanting to twin it, do i have to find the exact same card or can i just add another 7800gt 256mb card

 

Doesn't need to be the same manufacturer, but does have to be another 7800gt 256mb. Only think to check is the core/mem clock speeds, as the card in the first GPU slot will set the speeds, so if your current one is an factory overclocked version, and the new one isn't, put the slower of the two cards in the first GPU slot.