My attempts at grabbing videos are bleary and the cursor sort of lurches around. I understand some of that has to do with the fact that I'm editing in iMovie (does this go away in Final Cut Pro?)

Anyway I'd be interested in hearing how the pros make such crisp looking screencasts.

What pros are you referring to? Anyways, to actually answer your question, there are three big things that determine how good a recording is going to be.

1. Capture Area – A good recommendation (in terms of video capture) is to record and work at the lowest feasible resolution. For example, you might only need to record a single window of an application instead of all of the application's windows. If you need to record the entire screen, change your resolution to 800x600, or even 640x480.
2. Processor Load – What application you're running makes a huge difference. For example, if a video game is only running at 3-4 frames per second, that's all it's going to capture! Additionally, quiting other applications to free up memory may help. And yes, iMovie is slow. I'm hesitant to say that the problem will go away with Final Cut Pro/Express, but they are much faster and nicer to work with, at least in my experience.
3. Record Framerate – Just because it's set to 30fps doesn't mean it's going to actually capture 30 frames per second. In fact, I've found that setting it higher sometimes reduces the framerate. It ends up just making the processor load problem worse. So try reducing the FPS setting in Snapz to 25, 20, 15 or 10.

A few other factors are things like RAM, hard drive load/setup (Snapz can only write data as fast as your drive can), color depth, what version of OS X you're running, etc. I've found Snapz Pro X to be pretty well optimized in terms of processor speed, so the biggest determining factor there is really the application you're recording.

[edit - on the note of hard drive load, try moving your iMovie project to an external drive (or a separate internal). Snapz needs to write massive amounts of data to the boot disk, so if you're also trying to read massive amounts of data (iMovie), it's going to slow things down. Loading the iMovie project from another disk will reduce this load drastically.]