Yes, you can utilize the new audio formats with a PS3. The PS3's only option is to decode these new formats and send it as LPCM, as it can't bitstream them. As long as your receiver can accept multichannel LPCM over HDMI, you're fine. And yes, you can still use your SD TV until you get an HD display.
I wouldn't worry about storage unless someone is going to use it for gaming a lot. There are slight differences in features depending on the model with regards to SACD playback and backwards compatibility with PS2 games. If neither of these concern you, I think any model of PS3 should do.
Now someone else with more knowledge can fill in the gaps.
