When you start talking memory with DDR and SDRAM you really have to think about the processor & i/o devices installed on the machine as well.
There is no point in having 3GB of ddr memory on a Pentium 2ghz celeron with a 2GB 5400rpm drive. Its just a waste of memory.
However with todays technology rapidly expanding, DDR enhaced with dual + heat spreading technology makes it a killer combination.
Any 2ghz+ xeon or h/t processor with 4 sticks of 512MB DDR PC3500 corsair memory and a nice scsi drive will just tear up the server scene. (I built one of these a few weeks ago for a client. This puppy flew!