Make aditional backups without overriding previous:
There are two ways, the first one is obvious - use different tapes :-). The second one is more complicated.
First of all, some background information: each tar command writes a single file to the tape. For each SCSI tape there are two devices: /dev/stN and /dev/nstN. I see that you use the first one. The difference between the devices is that each time you perform an operation on it, such as read or write the file, it automatically rewinds. After you write the first file, it rewinds to the start, and when you write the second one, it overwrites the first one.
You should use the /dev/nstN instead, the first "n" is "non-rewinding". After you write the file, it remains at the end of file, thus when you write the second file, it appends to the tape.
In order to restore specific file from the tape with multiple files, you should use the "mt" command. Generally, you have to place the tape to the start of file you wish to read, and then use the standard tar command. I do not remember the exact syntax of mt, so you should read it's man page youself.