Not a great idea. Some boards will freak and not boot -- sounds like you're lucky on that score.
Some will run them at mixed speed (there used to be memory test programs to help you find your "fast spots" in memory, but that was a LONG time ago)... most likely you're just running everything at the lower speed.
Now, having a gig to play with might very well offset the speed loss. You're running something fairly current as your OS on that rig, yes? What the hey, 266 ain't exactly slow.