Err, I did say that the RD300SX is OK
as a keyboard, but whether it
suits your need, it's another question. I also said that the rhythm tracks may not have accompaniment and variations which may not be what you wanted. Also, I think we all suggested not jumping into getting new hardware too quickly. What I'm suggesting is not going into a keyboard until you have tried ALL options - whether be it other models of perhaps even not getting one at all. Unless you urgently need it, then of course it's OK. Otherwise you may find others more suitable to your needs - you won't know until you've tried them out. The key is not whether the RD300SX is a good keyboard - there's not doubt that it is. The key is what best suits your needs.
But nevermind. You like the piano sound - that's good. The most important thing is you like it and it is eventually useful for you.
Midi and the USB connector DO NOT transmit audio signals. They transmit midi signals which is NOT audio. Midi signals, as I explained, is the system messages used by electornic instruments. They transmit info like notes, velocity, duration of notes, controller changes (mod wheel, sustain, pitch bend, expression pedal...), aftertouch, program changes etc. The only thing that transmit audio from your keyboard is the audio out (L and R) and the headphone jack.
For speakers, you can try the home theater amp. It may give sufficient definition. It's just a hassle that everytime you want to play, you have to turn on your theatre system. PC speaker - well, it depends on what the brand is. But generally speaking, it is pretty low-fi. My suggestion is
DON'T get any monitor speakers yet - try out your existing speakers at home. If it works for you, then continue with it. Unless of course, you don't mind spending more money...