I have three moultrie I40s and a Bushnell Trophy Cam.
If I switch these to take videos instead of pics, do the batteries last as long? Also do the memory cards fill up faster?

I am guessing yes and yes.

You are half right. A 1 gig card will only hold a few videos and it fills up so fast, you don't have time to run the batteries down. I've only had my Moultries in the video mode once and I think it took like 10 or 12 videos.

Blessings.....Pastorjim

My Scoutguard in video mode shooting at highest setting of 640x480 at one min vid saves a pic of about 23.4mb during the day and a little smaller for IR video.
If you do the math it will save about 80+ videos on a 2gb card.

I think You can set it on Low-Res Video and High-Res Video,the Low-Res video allows more video to be stored in memory but the quality of the recording is less than the High-Res video's.You may want to use a higher gig card if you plan on using your camera in video mode because it will use up more space faster.Your Moultrie Instruction Manual will have the different spec's on how much Pictures/Video's (Time) is stored on Internal Memory,and a 256MB SD card.You can also go to the Moultrie Site to see the info there on your model of Camera.

www.moultriefeeders.com