Start by checking mobo specs to determine what type slots are used. For this particular mobo:
http://www.gigabyte.com.tw/Products/Motherboard/Products_Spec.aspx?ClassValue=Motherboard&ProductID=1883&ProductName=GA-K8N%20Pro-SLIThere are two X16 PCIe (PCI Express) slots which support SLI mode (2 cards coupled).
To determine what's available on Newegg, just search for video cards which meet your criteria. Here's all PCIe 2.0 X16 cards (PCIe 2.0 is latest spec and higher performance cards will be built to that spec. PCIe 2.0 cards work in PCIe 1.1 slots but bandwidth is limited)
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010380048+1069633099&Configurator=&Subcategory=48&description=&Ntk=&SpeTabStoreType=&srchInDesc=Then you can narrow search by GPU, GPU manufacturer, SLI support, or any other pertinent criteria.
Here are GeForce 9 series cards:
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010380048+1069633099+106792522&Configurator=&Subcategory=48&description=&Ntk=&SpeTabStoreType=&srchInDesc=And search further refined to GeForce 9800GT cards:
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010380048+1069633099+106792522+1067942536&Configurator=&Subcategory=48&description=&Ntk=&SpeTabStoreType=&srchInDesc=In this case, it would help to know the budget involved. PCIe cards range from around $25 to over $500. Some good moderatley priced cards were recommended in your PC build question. For higher performance, take a look at nVidia GTX200 series cards and Radeon HD4000 series cards -- particularly 4850 and 4870 cards.
