How do I make the left half of a video clip show an FX and the right half to appear without it?

Put the clip on 2 tracks. On one track, use the frame size keyframe tool (can't think of the right name for it) and cut the screen in half.

On the second track, do the same but for the other side.

Do your effect on one track or the other.

Easy as pie.

There is another way thats easy as well.
On the left of the Preview button, ( draft, preview , good, best etc )
There is the split screen view button, choose which side you want to bypass all FX.

Thanks Perrone...that should do it, I've been up for awhile and couldn't think!

Gerald, not a bad suggestion you have, except I should've explained I did not just mean for preview purposes...as I think the split screen thingy is only for preview.

LOL, in that case, what Perrone said.