As far as the toothbar, any of them will do. I bought the Woods from Carver Equipment in NC. I am very happy with it.
The toothbar was originally for a 54" bucket on a Deere 670. When I got my 790 with a 61" bucket, I purchased a foot of the steel flat stock (it's not actually flat...beveled on the front edge) and a tooth and extended the toothbar to fit the newer bucket.
As far as shimming the relief valve...there have been quite a few posts on this so do some research on TBN before you adjust the pressure. There can be drawbacks to this. For one, you'll invalidate your warranty. You can blow lines or bend the cylinder rods if you boost the pressure up too much.
Since you admit you're new to tractors, I strongly recomend you not doing any shimming until you've more hours and learn to operate that tractor. You may find technique is more important then pressure.
Definitely get the tooth bar. It really helps when digging into a pile of dried out clumpy dirt. But it won't make your 790 an industrial or construction type loader.
