Virtual box is a good way to test different operating systems but if you're limited on ram, have a low level processor or graphics like my acer aspire 3620 you will not experience the true benefits of a true linx Os. On my acer the perormance is not there, but my amd machine is great.
In fact on my acer debian runs better than the windows that is installed. I rarely use wins.
If you want to download linux, just download it.
If you want to install in virtual box just save the file setup your machine and point virtual box to the image to load.