What you should do depends on what you want to accomplish, and what your budget is. If you are wanting an improvement in sound quality, look at speakers. Go to all of the local audio stores and listen to as many speakers as you can stand to listen to, of as many different types (e.g., ribbon, horn, dome, whatever) as you can, and of as many different brands as you can find. Then pick the ones you like. After that is done, then you will need to consider the amplification needed to drive the particular speakers you have selected. You may not need any additional power at all, depending on what you choose.
Nothing else matters nearly as much as speakers for sound quality.
I personally would not bother with the Oppo, until your current BD player dies, or you find that you need something that the Oppo does that you otherwise cannot do. And I do like Oppo, so it would be a fine choice when you need it. (In my case, I have a Panasonic BD player, but plan on buying an Oppo when the Panasonic dies, or if I really need something that the Oppo does. But it is not a top priority at all, and I already have speakers that I never plan on upgrading.)
As for the subwoofer, SVS is a good value. If you spend much less, you will get less.
At this point, I personally would not buy a receiver for use in my main home theater that lacks HDMI and decoding of all of the current audio formats. It is a lot easier to set up if you have such a receiver.
As it happens, I coincidentally own a Yamaha HTR-5790 which I use as a secondary "home theater," and it is a nice unit. But I would not want to be stuck with it for my main home theater (though I could make do if I were poor and desperate). To me, HDMI is essential at this point.