Todd, thanks for your response;
I should have mentioned that I am using CS3; I do not seem to have any of the Separate Dimensions ... is Separate Dimensions a feature unique to CS4?
Yes, Separate Dimensions is new in After Effects CS4.
aha - thanks though;
You can also hold x value by writing the following expression to the position property:
x = 300; // value of x position y = value + ; z = value + ; [x, y, z]
You could also tie the x = to a slider. The value + statement makes it possible to set keyframes for values not set to a specific value.
I actually used this not too long ago when I was running a demo.
For CS3 there is an animation preset that comes with AE and offers a similar functionality. It rewires the position values to expression sliders and would allow you to dial in a fixed value for X while animating the others....