As a general rule, higher rated DDR DIMMs are designed to be backward compatible with lower rated DDR DIMMs.  But when you mix memory speeds, the memory will always default to the lower speed.  So if you want to upgrade to PC2700, you may have to replace all existing modules to get the optimum performance from the new memory.  In addition, always bear in mind that adding a faster memory does not always make your system faster.  You have to consider other factors such as processor’s FSB which dictates how fast system components would normally run.  For best performance, it is always practical to follow the recommended ratings stated in your technical reference manual.