It may be a little better, but don't expect blazing performance differences. The extra bandwidth makes the biggest difference when you have a video card without enough onboard memory, so that it has to access main memory for texture caching. It's probably better to run the games at lower resolutions in this case so that it doesn't have to access main memory, as it's so much slower than onboard memory that it would reduce your framerate. AGP texturing is primarily useful only if it means the difference between playing a game with low performance, and not playing it at all.
Your current board by the way has an AGP4X slot on it, not 2X. Even less of a performance difference going to 8X from 4X.
If you're playing games where you're CPU limited at this point, then you may see larger performance increases, but in games where the GPU was the limitation, you're not going to see more than a few percent difference most likely.