If you read through my Hearoid project you'll find that keeping as stock as possible was the main goal, which included the arms... and I had a lot of trouble with the arms.

The wrists can be rotated without any problem with a micro servo, one of the small 9g ones (TG9e). These fit inside the wrist nicely too however with the claw there would need to be a fair bit of modification made if you want it to open and close too (and a servo would need to be on show).

The elbows need a high torque servo to be able to lift the lower arm and hands. I used Tower Pro MG90s and have them powered externally with 6v to give them the most torque possible (reports are that they can take 7.2v but I didn't fancy testing it out). These require a bit of modification to the arms to get them to fit in, I found that cutting away at the existing pivot was the best solution however the joint was very flimsy so needs strengthening too.

The shoulders need high torque too, I used MG995s (standard servos) to lift the whole arm, also powered by 6v externally. They are borderline OK but holding the entire arm out straight will cause them to buzz, you may be better off with a worm gear for this one. There is plenty of space inside the torso to accommodate the shoulder servos.

I covered it all, including the failed attempts in my Hearoid project, have a read through, there are plenty of photos too :)

Edit: I just read the title properly... The above works for standard Omnibots, I'm not sure on the 2000s.

Thank you very much, I will look you Hearoid Project.