Even if your motherboard is designed for AGP 4X, an AGP 8X video card will still work.
The speed will be clocked down though to the speed of an AGP 4X video card.
If this doesn't bother you, then I see no problem in buying an AGP 8X video card.
Don't buy that video card though. Since its a SE version its been clocked down.
If I were you I would go for a Radeon 9600 Pro or a Radeon 9800 Pro as well.
dan