The difference you will see would be a result of the specifications of the new vs old graphics card, not as a result of AGP 8x vs AGP 4x. Changing graphics cards on the basis of AGP speed makes no sense.
Barton 2500+, 512MB Corsair Platinum XMS 3200 CL2, Radeon 9700, WD Raptor 10,000 rpm S-ATA HDD, Asus A7V600, Enermax 460W SilentPlus PSU.