Forget about DPI and PPI settings in PS. They will confuse you until you understand the concept of resolution.
If your image is 1000 pixels wide and you print it - or have it printed - 4 inches wide, the resolution is 250dpi (1000/4).
If you take the same image and print and print it 5 inches wide, the resolution is 200dpi (1000/5).
Regarding color space, I wrote this post on an underwater Photo site I am a moderator on and I think it may help you so I will repost it here:
I have been asked to provide a sticky thread document explaining color space because there have been some questions about colors looking different online than in the photo editor. I would never identify the person that asked me to do this but I will say that SHE has been getting questions about this, is an administrator of this forum, has super powers, and will always be obeyed eventually so it’s best to get this over with.
I next want to disclaim anything I say by stating point blank that I know little about the subject and will tell it like I think it is but will happily accept corrections to any errors I make.
The purpose here is to try to explain only the color spaces we will be concerned with while learning digital photography and to do it as simply as possible.
1) All of the color we can see (aptly call the visible color spectrum) are not all the colors we get when we use different digital devices.
2) Color spaces are ranges, or gamut of color that fall within the visual color spectrum
3) No devices, such as monitors, printers, and cameras can reproduce what we see – yet, so we have to work within the spaces, or boundaries that these devices can handle if we expect to output to them with acceptable results.
4) Most printing devices use 4 basic colors of ink to create all of the colors we see on the printed page. They are Cyan, Magenta, Yellow, and blacK, or CMYK.
5) Monitors mostly use Red, Green and Blue (RGB) ray guns to create all of the colors we see on our screen.
6) The 2 color spaces we will concern ourselves with here are sRGB and Adobe RGB. Of these two, AdobeRGB has a wider gamut and so contains more information. It is also the closest to the CMYK range so is best for images that are destined to be printed. sRGB on the other hand, is a narrower range of colors. This space is used more by Internet software and is usually more universal.
For our purposes, it is generally best to have our files set to AdobeRGB. This is because we may want to use our images online or as prints so we may end up using both spaces. If you start with the wider space (AdobeRGB), you can always convert it to the narrower space (sRGB) later with little or no noticeable change to your colors. The reverse is not true.
Our first encounter with color space in our workflow, may be in the camera. Depending on the camera, we may be able to select that our jpg files be saved as sRGB or Adobe RGB. Or we may have one or the other with no choice. If your camera only saves files in sRGB, you are done thinking about it. Keep them in that colorspace throughout the workflow. It will do you no good to ever change it to AdobeRGB since those colors were never captured in the first place.
If you have a choice, set it to AdobeRGB even if you are shooting raw. This won’t affect the raw file but it will affect the review thumb making it closer to what you will see after you convert the raw file, although I doubt it really matters. The only time you would want to set your camera to sRGB if you have a choice, would be if you are saving each image in RAW and jpg (some cameras offer this choice), and you expect to only use the jpgs online only. This way you won’t have to change the space on your jpgs but you can still use AdobeRGB when you convert the RAW file.
For raw files, be sure to set your converter to output to AdobeRGB so you will get the wider gamut for editing and printing.
Remember to convert your AdobeRGB images to sRGB before saving them for web use. This will keep them from looking different online than you saw them in your editor although sometimes you may still notice a slight difference. In Photoshop CS2, this is done by going to edit/convert to profile.
Oh, one more thing: don’t make the common mistake of converting an image to CMYK for your desktop printer. Even though it will use CMYK to make the print, it does it’s own conversion and expects an RGB file. Sending it a CMYK file will only confuse it.
Hope this helps...