Welcome to AT.
If I was in that situation, I'd certainly stick with AGP.
I recently moved from a 2500+ barton to a 3200+ A64 and kept my 9800pro.
If you can sell your 6800 for close to what it would cost to get a new one, then it might be a good idea, but if you're going to lose a significant amount of money selling it, I wouldn't bother.
If you're happy with the 6800's performance, I'd say stick with it. When you want to upgrade down the road, you could sell the AGP 6800 and A64 AGP board as a bundle and get a PCI-e board and card.