You can mix DDR ram and have it work at the slowest speeds. If you have some 2-2-2-6 PC2700 ram and some 3-3-3-8 PC3200 ram, it should work together at 3-3-3-8 PC2700.
While it should work there can be problems, including the memory controller not reading the SPD values correctly. Or it can assign values that won't work. Running only one stick of ram and manually setting the values should fix this.