There is a supply and demand for higher performing cards, and the supply is just not there. Neither Nvidia or ATI are going to give you a performance at a decent price, there is just to much money to be made. AGP isnt dead, its a moneymaker.
Of the reviews I've seen, I can't even see a dedicated graphicsaholic like yourself being sufficiently impressed by the switch to the 7800GS. Sure you may be able to overclock substantially but you could equally be pushing your 6800GT a little harder I'm sure. I'd at least wait a week or so and see what a) the buyer reviews report about OCing & unlocking and b) what happens to the pricing. I bought the BFG 6800GS on the day of release for $190 and since then it has bounced around between $195 and $230 on Newegg. If you are planning ahead for future graphics engines then just wait until the games are actually released. Unless you have money to burn, in which case send me your 6800GT so I'm halfway there to a decent PCI-E system!