First you have to plan on what type of system you plan to run. If you intend to upload video files from a camcorder then capacity for OS drive will come into play. Some will take a good sized single drive and see a second storage partition for backing things on that in case the OS primary needs to be reformatted or recreated at some point.
With Vista a good start with prices on drives having come sharply in the last few years would 160gb or larger depending system. You could run Vista on a drive as small 10gb. When adding softwares you will then chew space and need a larger drive for obvious reasons. Most know look at 250gb, 320gb, or even 500gb for capacity.
Downloading and saving files to any second drive won't have any direct effect on the OS installed on the host. It's when going to use or open files with the OS in case of any virus or spyware contained in a download you would see problems.
Most know will look over the selection of large capacity drives now seen for under $80- and end up with a pair of 500gb models if they need the drive space like those seen at
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010150014+1035915133&name=SATA+3.0Gb%2fs
For simply having a smaller drive for the OS and on a budget the 80gb model here serves as the OS drive.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822135106
For those looking for performance and ready to spend more the WD Raptor drives see the faster 10,000rpm models.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136033 Gaming systems are the type those go into for the most part.
With dual booting XP along with Vista here two separate internal drives are used along with an external 500gb model for longer term backup of files for both versions of Windows. The larger internal drives are in place for working with video occasionally where drives can fill up fast.
Here capacity even for XP saw the need. For a basic system and simply downloading some files and seeing them stored on a second drive you have a number of options but leave some room for adding programs later.