8x AGP is backward compatible with 4x. As you probably know, AGP started being phased out some time ago. For use with your motherboard, look for an AGP version of 6600GT or 7600GT. A higher card such as a 7900/7950 will be severely bottlenecked by the rest of your system and will be a waste of money unless you can get them real cheap. 8000 series are not made in AGP version.
Even an FX5xxx or GF6200 card will be a big improvement on the TNT2.