Not too long after the entrance of ATI's X1900GT graphics card into the market, ATI have decided to adjust the core and memory clock speeds from the original 575MHz / 1200MHz to 512MHz / 1320MHz.

TechPowerUp! and HKEPC explain why.

ATI plans to change the specification of Radeon X1900GT, from 575MHz Core/1200MHz Memory to 512MHz Core/1320MHz Memory. One of the reasons is to clean up the stock for preparing the upcoming RV570 based Radeon X1950PRO in October. ATI Radeon X1900GT is based on the latest R580 core with 12 Pixel Shader Pipeline, 36 Pixel Shader Processor (ALU) and 12 ROP. In the previous specifications, the high clock speed has a duty to compensate the performance loss due to disabled Pixel Shader compared to X1900XT and XTX series of video cards.