Walk Away the Pounds are the only workout videos I have done, and while I love them, I would really like to take things to the next level and try something different. At the same time, I have weak knees and don't have very good range of motion. Which videos would be easier on my knees, Tae Bo, Core Secrets of Turbo Jam?

I've done the Kick Butt (kick boxing videos) and just recently purchased the Turbo Jam set.
I love the Turbo Jam (and they have variations for people who want low impact). One of the ladies who does the low impact actually lost 75 lbs. She had knee surgery so she keeps it low at all times

Core Secrets probably isn't the way to go for weight loss, I would say Turbo Jam too, unless you don't like dancing. I found Billy really hard to follow. I started with Kathy Smith's Project You and got great results.

Have you thought about using 1, 2, or 3lb. hand weights while doing WATP? It intesifys the work out trust me!

I moved up to Tae Bo after using the WATP DVDS. Its a more intense workout but seems to be easy on the joints!