the difference is bus speed in my point of veiw they conflict each other and result is crashing the system and some time stuck probley system protectsion error .use the both ddr ram with same buss speed
They would work together if the bus speed was '333MHz' or below.
There is nothing wrong with underclocking the RAM as long as the BIOS settings don't try to set the bus speed from the RAMs Serial Presence Detect EPROM (called SPD in most BIOS). That could possibly cause problems, I don't know.
It probably would run with SPD enabled but at the speed of the slowest DIMM if mixed 2700/3200 units were used.
I have had PCs running PC3200 memory at PC2700 speeds (including this one when I had a Athlon XP2500+ (333) CPU and which now contains a XP3200+ (400)). I have an Athlon XP1800+ (266) machine too which at the moment has mixed PC2100 and PC3200 RAM.
RAM dosen't need to work at the speed on the label, it will work at lower speeds.
Run mixed ram on several different systems over the years, some like IBM's don't like it others, usually home built pc's don't seem to mind as much. As leggo rightly says it will all default to running at the lower speed but shouldn't give you any headaches. I've got bucketloads of ddr @ home so if you get stuck give me a shout.
The problem with RAM comes when mixing types i.e ecc and non ecc, this will cause instability so don't do it.!!!