The x700 is is for a PCI-EXPRESS slot not an AGP slot. It is a completely different slot not available on the Dimension 8300 (I have this model). You cannot use a PCI-Express card in an AGP slot because:
1. It will not fit correctly.
2. Your tower does not have the slot.
3. If you try to put the card in you will either break the card or blow your power supply.
Actually, sorry I didn't realize you wanted the AGP 8X version. The "x" in x700 means express and that card was made at first for PCI express. I did not realize there was an AGP version, and the AGP should work with your computer.