I realize that I come late in this discussion, and take the risk to add to confusion. But I don’t agree with the advice of @Ed_Fritz. I worked for many years in R&D of a large printer company and can say that I have some experience with colour.
I own an Eizo monitor, that covers the recommended 99% of the AdobeRGB colour space. I work in AdobeRGB for post-processing. If I convert my images to sRGB and compare them side by side with the AdobeRGB images, there will be small differences, but I’m certainly not blown away. The main part of the spectrum that is affected are the light greens, however, so not unimportant for nature photographers. As soon as you print, the world is very different. Traditional wet-chemical photo papers deliver beautiful prints, but do not even cover the sRGB color space. Inkjet printers have a much larger gamut, but the colour space depends strongly on the inks. Dye inks generally have a very large gamut, pigment based inks are often weak in the light green region (where AdobeRGB has its advantage compared with sRGB) because most “cyan” pigments are more blue than cyan. If you want the best in that part of the colour space, you need a printer with a specific ink for that part of the colour space, so not only CMYK (or CMYK with “light inks”, which are just diluted inks used to improve graininess in light areas, not colour space).
If you know the ICC profile of your printer (inkjet or traditional chemical from the print provider), you can get an idea of the print result when you use the soft-proofing option in Photoshop.
ProPhoto is a very large colour space. If you do your editing in this space, many of the levels are not used, since the gamut of your photos will be much smaller. IMO this is not the best choice. You MUST do your editing at 48bit images (that’s a good advice anyhow) if you decide to use it, otherwise you will lose too much when you return to AdobeRGB or sRGB, e.g. for printing. Most monitors will not be able to display a large part of ProPhoto.
In the past, I calibrated my monitors with an X-Rite i1, at home as well as at my job. They are very good, but two of them were defective within a few years and were unreliable after that. If you contact X-Rite about that, they send you a small diagnosis app and can confirm the status of your device if you communicate the results. When the one that I used at home was broken, I decided to buy the dedicated EX3 device, that works with Eizo monitors only. Till now, it is fine.
The advice to keep your monitor brightness low, as mentioned by several, is wise if you intend to print. The room lighting should be at a low level as well.