1) It's not so much the cable or the connections per se, it's that you've got (presumably) balanced circuitry at both ends that provides a benefit. Rather than have one line signal, you've got two although they're of opposite polarity. Numerical benefits are seen in a couple of areas. First, there is a reduction in hiss of approximately 3dB by virtue that it's balanced. Secondly there is an enormous reduction in noise pickup of 40 dB or more.
2) It can be, yes. Inexpensive sources of XLR connections abound and ready sources can be found at places like Guitar Center (they're all over the country, like Burger King). Some time back I recall someone posting a link for what appeared to be pretty inexpensive cables. If I can locate it, I'll repost it.
An additional benefit of XLR is that the connector and connection itself is much more robust than RCA's.
So will you realize an audible benefit? That really depends if you've got audible noise to begin with or exteranlly generated fields that are inducing noise in your RCA's. Generally, XLR's have their greatest utility in very long distances. Seeing as you've got it, regardless if there's an audible benefit or not, I'd use it. It seems to me the prudent thing to do.