The value of a SSD is primarily the fast random i/o times. perhaps 50x faster than a hard drive. That is what the os does mostly.
The benefit is not just boot times, but everytime you do I/O Under the covers, windows does things like logging, pageing, catalog search, etc.
Everything you do seems quicker. The SSD is best used for the windows drive.
The sequential performance is 2-3x faster than a hard drive. Yes, it can make level loads faster. Virus scans go quicker.
The easy way to convert to a SSD is to use cloning software. It will make an exact image of your hard drive on the SSD. If your hard drive is sufficiently small, this is an easy way to convert. Intel provides a free ssd migration tool based on the Acronis true image cloning software. If your hard drive space fits on the SSD, great. After migration, clean off the hard drive, and use it for overflow storage.
If it does not, then you are best advised to just do a reinstall on the ssd. Windows easy transfer will export all of your settings and files, and you can import them all, or selectively to the ssd. Apps will have to be reinstalled.
It sounds like you are contemplating a new build. If so, I suggest you consider deferring a hard drive purchase, and build initially on a SSD. A 120gb ssd will hold the OS and a handful of games. Buy a SSD later when or if you need more space for overflow or large video files.