Your MoBo is probably AGP 8x compatible.
When a card says AGP 8/4x - it means it will work either at 8x agp or 4x agp, depending on your board - if your board is 8x AGP compatible, the card will run a 8x - it's that simple.
The (#)x denotes the bandwidth of the AGP interface...
AGP 2.0 is a 1.5V AGP slot capable of 4x,2x and 1x speeds.
AGP 3.0 is keyed like a 1.5V AGP 2.0 slot, but it only uses 0.8V of power. It supports 8X, 4X, 2X and 1X speeds.
Don't worry, since you're probably AGP 8x compatible - you have AGP 3.0, that means that all AGP cards will be compatible and will run fine.
If you can give me the MoBo chipset version I could check to be sure that you can run the card at 8x, however, even 8x cards will work if you dont have an 8x AGP board, they will just work at 4x.