You can use either an HDMI connection or multichannel analog connections for True HD sound. As Seth=L says, it is easier to just use the HDMI connection.
As for the upgrade, it is probably not a good idea to upgrade now, given that your player has an internal decoder and what you now have allows you to do everything you need it to do (unless you have not mentioned some other relevant fact). Although if someone were buying soon, I would recommend that they consider the 663 instead of the 661 to be more secure of the future. But since you have no need of it, it would be a waste of money for you. In the future, if you need something more than what your 661 will do, there will be other things available to take care of you. And by then, you might need HDMI 1.4 or some yet to be invented connection that the 663 does not support. It is NEVER a good idea to buy something simply for a future need that is not needed now or very soon. The reason is, it is impossible to predict the future, and there may be some other thing that you will need that you cannot get now or that would not be on the model you would select. If what you have now does all that you now need or will need soon (say, in the next 3 months), buying for the future is simply a waste of money.