Definitions

sRGB

    The default setting for Photoshop 5. Not the best for print work
because it clips the cyans, and the blue and greens next
to the cyans in the color space. Better for images destined to the Web.
Blatner and Fraser in Real World Photoshop 5
recommend doing your editing in a
space larger than sRGB, and then
converting to sRGB only when the image is finished.
 

Adobe RGB (1998)

    Has a color gamut that almost matches
the entire CMYK gamut. Worth considering
using.

Color Match RGB

    Color Match is based on the Pressview monitor
by Radius color space. The conservative choice
for print work. Color Match has a Gamma of
1.8 which is a small color gamut, with less
information in the shadows.

Monitor RGB

    The "work like Photoshop 4 option"
and the color space is tied to your
specific monitor, so in the long run
you better off letting the "Display using
Monitor  Compensation" feature handle
the translation to your specific monitor.

Custom RGB space

I have customized my working space
to Bruce Fraser's customized settings from
Blatner and Fraser Photoshop 5, known as
Bruce RGB. Bruce RGB is:
 

white point    = 6500K
gamma          = 2.2
red xy           = 0.6400  0.3300
green xy        = 0.2800  0.6500
blue xy           = 0.1500  0.0600
 

Rationale:

    BruceRGB has Red and Blue
that are the same as Adobe
RGB 1998, and the 6500K
white point is a good interpretation
of a real world display, and better than
the 5000K (D50) standard.