You only need it if you use 25x16 resolution. Dual-Link supports resolutions up to 25x16, single link supports only up to 16x12 and 19x12 with some interleaving (whatever that means). So as long as your monitor is 16x12 or below you are fine with single link card, if you need 19x12 single link would still work, although I would recommend dual link, anything higher and you need dual link card.