I'm not a yoga expert, but any exercise you do should probably be carefully chosen so as not to stress joints that are already causing you problems.  It's a good idea to gently exercise joints to keep their range of motion and reduce stiffness, but not to push too hard.

    So - it will depend on where and how exactly your RA is affecting you. 

    One thing that I enjoy is Tai Chi for Arthritis.  It's low-impact and at my own pace.  I got a DVD from the Arthritis Foundation and it's also available on www.taichiproductions.com.  But even this you may have to choose what's best for you, and leave out what's not good for you.  A doctor or PT might be of assistance, or your local chapter, if you have one nearby (they have good exercise programs).     

I like doing gentle stretching, have never been athletic (even before the RA was diagnosed). When I got the RA, I hurt so much at first, I didn't do nearly as much, and I lost the flexibility in my hips that I used to have, to sit cross-legged on the floor, etc. and to stretch out my lower back while my legs are stretched out on the floor.  But about a year and a half later, when I was on Enbrel, I was able to regain most of the ability, but not quite all.  This is not to say that you should thisparticular exercise, but just to demonstrate that your routine may depend on your background in exercise, and current medication, for instance.  It's very individual.