This article was published last week, but with more XBox 360's shipping soon(hopefully), you might want to weigh the pros against the cons on buying the $400 bundle, rather than the $300 core package.
"You'll be surprised about the differences between the two bundled systems so consider these things before buying. In no particular order here are some CONS of buying the CHEAPER system. (Believe me, I wouldn't pump the more expensive system if it wasn't a big deal, I'm here to save gamers money.)"
If backward compatibility is a must for you, start getting that $400 ready.