24 fps is for film, certain HD formats and certain SD formats. 25 fps is for SD PAL, 29.97 is for SD NTSC and 54.94 (60) is for 720 p HD video.

.psd format in sequences is the best format if you plan on using non-destructive editing tools like AE or FCP since you can also export the alpha and g-buffer channels for extra effects or corrections later on in post production.

Regarding render times, since I produce for both SD and HD clients, make careful decisions about sizes. Doubling the size of the render image requires four times the amount of rendering time. So, for a fast render, 90 seconds per frame for 30 minutes of animation at 30 fps means 54000 frames and 81000 seconds - 56.25 hours of rendering - all this with no render issues or re-renders. Using particle effects or special effects from within Carrara can easily multiply render times by a factor of 5-10. All this in addition to simply doing the animation work. 

I was talking to some folks from ILM at SIGGRAPH last year and the the scenes they were doing for Raiders of the Lost Ark were taking 24 hours per frame on their internal render farm of about hundred quad cores. The final rendering was sent out to a very large render farm so that it would  be completed in time. You can never have too much speed.