9 points the difference in 3DMark 01...

AnandTech has directly compared an ASUS P5E3 Deluxe (X38) with a P5E3 Premium (X48) - clearly illustrating the performance similarities between the two chipsets. Using a Core 2 Quad QX9650, and 1GB of Corsair CM3X1024-1800C7D 5-5-5-15 (DDR3-1333) memory, the difference in the SuperPI 8MB test was only 8ms. This continues in BattleField 2 with the X48 leading by 0.31 FPS. The only thing that separates the two chips, aside from FSB's, is the overclockability...

Our initial test results indicate that except for official 1600FSB support, there are not any real performance differences between the X38 and X48 chipsets at stock or mildly overclocked settings. This does not surprise us, as Intel's chipset guidance outline has not indicated anything differently. It was not until we started pushing both boards that we could tell a difference, even then it was minor for the most part. The X48 based Premium board did allow us to reduce CPU and MCH voltages when compared to the Deluxe board. This could be a combination of factors ranging from BIOS and board design to improvements in the X48 chipset. In early testing, it is proving difficult to isolate the specific differences. We believe it is a combination of BIOS design and the X48 chipset. However, it appears on the DDR3 boards that the user will not be able to take advantage of these minor differences unless they are really pushing the board. This will be important for some but for the majority of users it will not matter which chipset is used.