You cannot install more than one operating system, only one. I recommend Windows 7 since it is widely used throughout the world and is fairly simple to navigate around, and user. Windows 8 is also the updated version of 7, but is a bit harder to use since they changed the interface, and added a lot new options.
It depends on the processor of your computer, plus if there's enough memory to do so. Keep in mind that changing a new operating system on your computer may slow down the computer because it needs to get adjusted to the new software and whatnot. Another thing is that it depends on which operating system will your computer support. Make sure you determine the optimal operating system that your computer will likely to operate properly without having any lag or slow down. I think having two different operating systems are alright as long as your computer has enough internal memory and a good processor.
You can have more than one OS because I currently have it. I have Windows XP and a Ubuntu installation currently with no problems. Though, if you system can't handle two OS then you have to delete your old partition. Just remember that any files you need have to be saved somewhere so you won't lose them!