the difference between DDR 266 (aka PC2100) and DDR333 (aka PC2700) would hardly be noticable, but DDR667 (aka PC2-5300) is another type of RAM (1.8 vs. 2.5 V) and runs mostly as dual channel memory needing another controller, means another motherboard ... and is of course much faster ...
you would notice a huge difference for loading time (if your harddisks can deliver data ...)
christian
and remember: only a CRAY can run an endless loop in just three seconds.