I'm not really sure about the I9000T as I've never even seen one.
If you only have 1 tar file, which one is it? If it's the PDA then there should be no reason why you can't flash that, and leave the existing CSC and MODEM files as they are. If it's the CSC or MODEM file you have already then you probably wouldn't see a lot of difference if you flashed one of those.
If I'm honest though, I can't really offer you any good advice on that particular phone because I simply don't know enough about it. I wouldn't like you to brick your phone because I said something would work when it won't.
Sorry I can't be more help.