The 9800GT equivalent is the 6670 but since you are upgrading, get at least and AMD 6850 or 7770 or a GTX560 to get remarkable performance improvement. Since the 9800GT fit to your case & power supply, you wont have any problem with these newer cards.
Thanks! I already had the original card replaced under warranty a few years ago. I wasn't sure which way to go from here.
I appreciate the reply.