Most significant: A8V has an AGP slot, the A8V-E uses PCI-E (depending on
the video card you have now, you may be forced to switch to PCI_Express).
Next is chipset:
A8V uses K8T800 chipset (dual core CPU-compatible with BIOS update)
A8V-E uses K8T890 chipset (*not* dual core CPU-compatible)
If you're going to the bleeding edge, it appears that the NF4 chipset is the
better way to go. (A8N-SLI series)