As sure as snow in winter or sun in summer, AMD has yet another refresh to its graphics card portfolio this spring. The Radeon HD 6790 is only a couple of misplaced digits away from the far more illustrious HD 6970, but you should be able to tell the two apart by another, altogether more significant spec: the new mid-tier card retails at $149. Predictably, its performance offers no threat to AMD's single-GPU flagship, but the 6790's 840MHz graphics and shader clock speeds plus 1GB of GDDR5 running at an effective
