Unregistered SDRAM limits the maximum frequency you can run SDRAM reliably with out having to introduce latencies, the reason is that more chips connected to the memory bus the greater the capacitance load.
Higher capacitance loads require more current to drive the signals to the valid voltage levels. So for the best memory bus performance, the less DIMMS the better (we've seen numerous examples of this, where a board will run stable with one DIMM, but lose stability once more DIMMs are added). Of course, less memory may result in better “benchmark performance” for your memory subsystem, but if you don't have enough memory for your applications then there will be an even bigger performance hit on the performance of your whole system than just running your memory at CAS 3.
So if you need more and better memory performance it’s best to select a DIMM with the highest memory density supported by your motherboard's chipset (check your motherboard manual). This results in less chips connected to the memory bus, therefore your memory bus will see less capacitance load.