the reason why that's so expensive is 'cause you can overclock that to a near 7800 GTX.
Says who? This is AGP we're talking about here, not PCI-E. As far as I know, nobody has written a review on the AGP 7800GS or mentioned any overclocking results, so it's too early to claim what it is or isn't capable of.
Having said that, there is no-way this card is worth nearly $400 dollars.
Based on the stock specs of the AGP 7800GS, other then the core size, this card is the same core speed and pipes as the AGP 6800GT. I doubt it's "twice as fast" as the 6800GS, especially if you unlock the 6800GS... yet the price would make you think it should be.
I'd like to see somebody test the AGP 7800GS and let us know what it is capable of... however review sites seem to have forgotten AGP users, and at $400 smackers, I doubt any budget-concious AGP-owner is going to be silly enough to buy one of these. If you're an AGP user, you're better off buying a 6800GS and unlocking it (for half the price), or try and find a 6800GT for a reasonable price.