We now support serveral classes of GPU boards, including X1600, X1800, and X1900 class GPU's from ATI. At the launch, we supported X1900 cards only. X1800 cards do not provide the performance seen in X1900's and so we strongly recommend X1900 class cards (although we now officially support X1800). X1900 and X1800 cards are actually quite different -- they have different processors (R520, R530 vs. the R580 [in the X1900 series]). The R580 makes a huge difference in performance -- its 48 pixel shaders are key, as we use pixel shaders for our computations.