Whats the best cpu that can go in an asus a7n8x?

Yuck Athlon XP are complete garbage not even worth looking into really. Just upgrade your board you will barely see a performance increase with any of those chips because they are all too slow to make a difference in today's technology.
 


You don't know what the OP is wanting to use this machine for. He may not need a huge performance bump and does not necessarily need to go out and drop a few hundred bucks on a completely new motherboard + CPU + RAM. I was in the exact same situation as the OP was a couple of years ago. I picked up virtually the exact same motherboard as he has (ASUS A7N8X-E Deluxe) with a ~1 GHz Duron in it for free. I already had some DDR sitting around, so I put it all together. That Duron wasn't quite cutting the mustard for the HTPC, so I managed to find a guy who gave me a Barton 3200+ for real cheap. The Barton worked- and still works- perfectly for that task. Dropping a few hundred on a new motherboard, CPU, and RAM would have gained me nothing since the old Barton was more than enough CPU for the job. The Duron wasn't and the Barton was dirt cheap, so it absolutely made sense to upgrade that motherboard.