That's not quite true. There are many, many boards that don't use the nVidia APU. The 8RDA (non-plus version) just has old-school 2-channel onboard sound. There are many other cheap nForce2 boards that don't use the nVidia APU, I just don't feel like going to look all of them up at the moment.
So unless you need the extra features of soundstorm and/or firewire or plan on using them during the life of your computer, it is not important to get the boards that use the MCP-T.
It's a matter of personal preference. Considering that the 8RDA+ is only about $10 more most places than the 8RDA, if you think you might use any of its features, it's certainly not a bad investment. Unless you already have all those parts individually, you'd spend more than that on the Firewire card alone, not to mention an Audigy or something similar.
Soundstorm is not a physical object. It's a set of requirements. 8RDA+ does not meet all of soundstorm's hardware requirements. But, it does have the nvidia APU.
Indeed. The only requirement from Soundstorm that the 8RDA+ doesn't meet is the required SPDIF output, which you can purchase separately. Everything else is there.