Colour Profiles -- It is possible to spend a lot of time and money calibrating equipment to absurd levels of precision. Since fudge is a basic ingredient of profiles and colour-matching, ICC profiles from different sources will give different results, and there is no way to tell whether you will like a profile without buying it and trying it. Fortunately, most people don't need to profile their printer at all and can get by fine with the default settings. Long ago Microsoft and HP proposed, and the computer industry adopted as a formal standard, a colour-matching technology that's simpler than the full ICC standard while still being sufficient for most people outside the graphics-arts industry. All devices are assumed to be able to produce a range of colours that will fit within a range or 'colour space' called sRGB. A standard set of numbers defines every colour within this space. All devices are supposed to interpret those numbers sensibly. It is the norm for photos on the Web, and most commercial printing services use it, so I've set my Mac to use sRGB by default (ColorSync Utility > Preferences pane > Default Profiles tab > RGB Default pop-up menu).
Most inks on most papers are limited to the range of sRGB, although some do exceed sRGB's range. From some inks a larger colour space defined by Adobe, called 'Adobe RGB,' allows more vivid colours. The difference is likely to matter in print competitions and some corners of the graphic arts trade, but it not clear to me that it would matter elsewhere. Using a larger colour space incurs a cost: it is likely to require 16-bit colour, which requires more storage and processing time...
...In any case, be sure to set the gamma to 2.2. That is the de facto standard for working with colour. The Mac's standard of 1.8 was intended to make a grey-scale monitor look like a printed page.
Years ago, when I was trying to understand color profiles and realizing how ugly this was, I corresponded with an Apple ColorSync engineer. He said: use sRGB everywhere. I've followed that advice. It works. More recent versions of iPhoto even honor sRGB.
I haven't set my gamma to 2.2, looks like I should. I do wish Apple would give up on the 1.8 gamma.