It's a nice up but you should research tomshardware.com to see if he has some machines with that difference. For laptops I head to reviews and benchmarks.
Same for desktops.
However since no make or models were shared I can only note that there could be CPU differences as well since that sets that rate in most if not all machines.
Is there a huge performance difference (speed-wise) between a 2x1GB DDR2 667MHz SDRAM and a 2x1GB DDR2 800MHz? How will Windows Vista Ultimate run on either of these? I ask this question because the difference in the price is too high.