The faster mem on the x1950xt will help it keep up with the 512mb model using slower mem. It will not, however, make it faster than the 512mb card. I've tested my x1900xt at various mem speeds, and the performance difference between 725mhz mem and 800mhz, for example, is only a few percent. In the end, given that the 256mb model costs considerably less and performs almost as well as the 512mb card, the x1950xt is a better value for the money.