Thanks
I will stick to sRGB
the graph above shows the entire spectrum of colours the human eye can see, sRGB is the range (known as gamut) that an average computer monitor can reproduce. aRGB shows more colours than sRGB as you know already (but still not anywhere near the full range of human vision), and as you can see cmyk can show more colours than sRGB in the blue/green and slightly less in red, but there is no 'CMYK', as CMYK is device dependant, i.e. every printer/paper has their own capability, for example the new canon 12 ink printer on glossy paper can produce more colours than a cheap 4 ink printer printing on cardboard.
Special 'wide gamut' monitors (which cost ££££'s and are generally used by people who don't ask such questions) can show the whole aRGB colour space, where as most monitors only show sRGB, so if you're capturing in aRGB and working on an sRGB monitor you will have colours you won't see but that's more of a problem in theory rather than in practice- think of it like tweaking colours while working on a black and white screen, you can't see what you're editing, although the worst that can happen is you might get some slight banding, but it's only a problem if you're making billboard sized images.
With raw, as POAH says your colour space is not fixed until you convert from raw, the aRGB/sRGB setting on your camera has no efffect on raw files, it's only when you export to TIFF/jpg that you set your colour space, in lightroom you can set the colour space used to render your raw files, by default this is prophotoRGB, which is even bigger than aRGB.
eventually monitors will be able to show the entire range of human vision, (the samsung xl24 is almost there) and eventually printers will be able to print every possible colour too, but for now we have standards like sRGB and aRGB to accomodate the majority
bear in mind that although aRGB has more colours, it won't make your pictures more impressive, or colourful, almost any image you've ever seen on the internet has likely been in sRGB, sRGB is plenty for most people, but if you have a wide gamut display then by all means go for aRGB and softproof to sRGB and see the little difference for yourself.
in short, files you send to other people who won't know what colour management is- sRGB, files for your own record/printing keep as aRGB to give you more printable colours
