you need cards that support SLi, they have a connecting piece that unites them
so 2 9800's wont cut it unless there made for it

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AMD 64 3700+ * 2GB (4x512) HyperX DDR500 * DFI LANPARY EXPERT * 5x80 Barracuda SATA Raid5 * 2 BFG 7800GTs in SLI * HP DVD-RW 630e * Lite-on DVDrom x16 *