RAM Considerations and Suggestions

Since I likely will have to replace this memory, could you suggest what you would think might be best, yet economical (I know, they don't often go together, but economics plays a big role here)? Would ECC, registered, or unbuffered make any significant differences? Would going to 2 Gigs (2 x 1Gig) be a big plus over 1 Gig dual channel (2 x 512)? There's not going to be a lot of digital stuff or gaming on this system. Lots of music though, and using the media center enhancements of MCE for home system components.

Thanks for your advice too.