I couldn't find an a7n8k on the asus website, but according to the a7n8x specs any AGP card out there should work.
"Best" probably isn't what you're looking for. "Best" will cost you $600 or more for a 6800 Ultra or the top of the line ATI card, since your motherboard is compatible with either in the AGP versions.
It comes down to how much you want to spend. For around $300 you can get a nice 6800 or x800, for $200 you could get a 6600GT or x700 pr 256 meg. Either of those options would run AH2 really well, and doubling your cost would not double your framerate or image quality. The payoff with the more expensive cards in AH2 is mostly how much eye candy you can turn on (FSAA) without hurting framerates, but other games would likely see bigger framerate increases with the faster cards.
I personally like the 6800GT because many of those cards can be overclocked to 6800 Ultra speeds without any modification, but the price point of $400ish is a bit high for most people. A 6600GT or 6800 would give you a better bang for the buck especially if it's just for AH.
I wouldn't go with the last generation (ATI 9800 or 9700 or nvidia 5xxx) cards because although they'll run AH almost as fast as the newest cards, they still cost within $50 of the newer cards even though they don't have all the newer hardware features that newer games are using and will be using in the next year or two. I suggest something between $250 and $300 in the current generation of cards and you would be set for a while.
If you are really on a tight budget, hit ebay or your local computer swap meet and buy a used/cheap geforce4 4600 or even a 4200. I wouldn't get an nvidia 5xxx series card even if it was cheap, because overall it was a pretty disappointing product cycle for Nvidia and they don't offer anything you can't get in a 4600 or 6600.
