Good luck with this.  I thought I'd throw my experience in to the mix, as well.

Mark has hit the frame rate info on the head, and his trick about up-ressing the footage is a great one.  I've also used a slight post production blur trackmatted with an edge detection in AfterEffects to soften up edges a bit.

One word of warning:  Carrara's alpha channel is a little weak when you're using motion blur.  Carrara's motion blur (use the vector blur) is quite good, but Carrara doesn't seem to know what to do with the alpha channel under the blurry parts.  For that reason, if I'm rendering something that I'm going to composite atop live footage, I render it against a pure green solid and key it using Keylight in AE.  The blur keys just fine in Keylight, and the built-in spill supression takes care of any odd coloration in the semi-transparent areas.

Hope that's helpful.

 - Dex