When my knees were at their worst, the elliptical put a big strain on them.
For cardio I got a recumbent exercise bike, which has worked out pretty good.
That still irritates the knees a little, so I moved to the upper body a bit for cardio. This impact might not be a good idea for some, but because of my really bad knee problems I got a speedbag to move some cardio to the arms. I use gel-padded gloves (striking with bare hands is NOT a good idea).
I know - it probably doesn't sound like a very smart thing - being arthritic and running out and buying an impact-based piece of exercise equipment! But I am not using this thing like a pro or using bare hands. The key to avoiding injury is to start VERY slow and careful, no matter what you are doing. It is much better to err on the side of non-injury than to do too much and set yourself waaay back.
I'm actually currently in the third week of P90X right now, and I just modify some of the moves to reduce knee impact and strain. I tried P90X about a year ago and had to HEAVILY modify almost everything because my left knee was swollen pretty bad at that time. It was really hard, I couldn't do a lot of the exercises, many I had to modify, and I couldn't be anywhere near as intense as a "healthy" person could in doing this program, but I still shed a lot of fat and lost about 30 pounds total.
Again, trying something like P90X probably sounds really stupid, but I actually bought the program just because of the wealth of knowledge it contains on body weight and hand weight exercises that you can not only do at home, but anywhere pretty much. I didn't want to commit to a gym membership and then be in too much pain or have too much swelling to actually use the membership - I went through that a few years ago - even tried to get a refund with a doctor note and such, but that is a story for another thread...
The funnest thing P90X contains is a really good cardio workout called Kenpo X. Not a lot of impact, but a lot of arm movement (punches and blocks and such). I was even able to do it when I had the really swollen left knee. And depending on how much pain you are experiencing, it is easy to reduce or increase the "intensity" of the workout.
Arthritis (poss. RA) - Doc scripts: Enbrel 50 autoinjector, Methotrexate 5 pills/week, Folic acid.