you would see a substantial increase in performance from even the lowest end AGP card, simply because the CPU would be able to use all of the memory bandwidth instead of half.
Intel Integrated graphics suck for 3 reasons:
1. it uses memory bandwidth that would normally be used for the CPU
2. they are 4-5 generations behind in design
3. Intel's drivers suck and almost never get updated or fixed
You're so much better off with an AGP card, it's not even funny.