Using an excercise ball chair at work...

I sit in a chair in front of a computer for at least 40 hours a week, thus; I have gained weight, I am a little worried about my cirrculation (blood clots are common in my family) and I have poor posture and an aching back. I have been working out 3-5 times a week, jogging at least 1 mile & lfting weights & doing crunches each time, which has helped alot, but I am still not satisfied. I have been researching a chair that is an excercise ball in a frame to avoid falling off, has wheels and arm rests. I've read that it improves posture, improves cirrculation, strengthens/ stretches muscels & can even help tone muscles. Has anyone tried anything like this or reccomend it? Thanks!