3D games are certainly the kind of applications that can appreciate fast system memory.
At a 400MHz FSB the system with DDR2-800 enjoys a 2 to 7% advantage over DDR2-600 with the same timings, which is a good result. Switching to DDR2-1000 SDRAM doesn't provide big practical benefits: the high bandwidth of that memory type cannot be fully utilized due to the limited bandwidth of the FSB. Note also the performance-affecting influence of memory timings. A simple modification of the timings in the BIOS Setup can provide a performance gain up to 5%.