I believe the 8k9a2 has 6 PCI slots (compared to 5) and the realtek 650 audio. I have this board in one of my systems, no complaints at all - its a great performer (for a KT400 chipset)...
Spend your money on an 8rda or 8rda+ ($85 and $89 at newegg), or any nforce2 motherboard for that matter.