That board doesn't <b>REQUIRE</b> AGP 8x, its just saying that it'll go up to 8x. But it is backward compatible, so you can run a 4x or 2x card in there too. That ti4600 will work.
<A HREF="http://www.anandtech.com/mysystemrig.html?id=24106" target="_new">My System Rig</A>
<A HREF="http://service.futuremark.com/compare?2k3=535386" target="_new">3DMark03</A>