In the end, since the 3 games I play don't use Physx, I decided to just install the GTX card, and keep the GT for a spare. I had not actually looked at them side by side before posting this. The GTX card is much thicker, and takes up 2 slots, as well as requiring 2 of the 6-pin power connectors instead of the single 6-pin used by the GT. So they are apparently very different in their makeup.
One note of interest to anyone reading this in the future. I had been running the "latest" Nvidia GeForce driver current at the date of this thread while running the GT. It was 196.62. However, after installing the GTX card, I ran into a problem where the displays would go to sleep once the Welcome screen was finished in WinXP. I tried everything I could think of, including even uninstalling my Triple Head 2 Go, and connecting a single display. But I wasn't getting any video.
Some research online revealed that for some reason, the 9800 GTX cards cannot use what at this point are the "latest" drivers. I read somewhere that a fellow had good luck with 185.xx, but unfortunately at the moment I tired to start looking for previous versions the whole Nvidia download site went out of service.
In the end, I had to revert to looking up an older driver on the EVGA site, based on a 5xxx series card, and found version 179.15, which is working fine. I know the 9800 series is getting to be an old card, and out of production, but this is the first time I had seen the lack of backwards compatibility on GeForce drivers recommended on Nvidia's site for a specific line of cards. Dialing in the 9xxx series points you to only one driver - 196.62. But that one does not work for the 9800 GTX.
Thanks to everyone for the help!
Zoandar