sounds like you have a revision 1 agp slot, if your board is much older then that is a definate but you can always check asus web site and the specs of your board.
if in fact it is revision 1 agp you might not have the voltage to run a gts, since they like to consume more energy than any V3, MX, or radeon.
I have heard of issues with gf2 mx's and older rev 1 boards before, but there are also those who get them to work flawlessly, however NVIDIA does require on their specs a revision 2 agp slot, which you can take with a grain of salt I guess since others are getting thiers to work.
You will have no problems with anything made by 3dfx
and the radeon, while it is designed for revision 2 agp and it is recommended on their page they have confirmed both in their faq and on their web page that it is quite backwards compatible with agp 1 allthough you will take a performance hit.
I myself am running a radeon on a revision 1 agp slotted bh6 and it is a noticable improvement over my v3 2000. However I might get more out of it had I a faster motherboard