choose the one with the right (or realistic) file size.

.dmg's are OS X disk images. SIT is StuffIT compressed files (mostly used for OS 9) and I'm not sure about the usability of gzipped files on Macs.

Speeds in increasing order are modem, cable, T1, T3 and Ethernet

Use the explore button to go to the saved downloads folder, then just double-click the file to unstuff or mount it.

You can't be sure the files are useful and virus free, but there are still no reported viruses for Macs, and I don't know if Virex's OSX virus scanner has even been updated to work with OS 10.4