Well I'll chime in on the side of balance, throwing myself to the wolves. While I agree that yes it's best to go with the source first, that is assuming that you DO NOT neglect the cables. Some will max out their funds on the source and never get around to taking care of cables. I have heard $7K cd players with $3K power cords run rings around $10K-$20K players and seperates with lesser cords. Don't laugh, I know some whose power cords cost more than their source, and swear by it.
So I would say to come up with a budget, the maximum amount that you think you will spend, and make sure not to ignore the cables. I won't go so far as to name a percentage, because synergy is too important. However, due to synergy, it is best to choose components first, as the cables that best suit them can be altered by your component choice.
I guess what I'm trying to say is that if you don't balance the system, and the system includes cables, you will be limiting yourself one way or the other.
Cheers,
John