The speed of it really wouldn't have anything to do with whether or not it uses up all 256mb of it's VRAM. The texture data of the game is what decides that. That's why Doom3's "ULTRA" setting was meant for 512mb cards because that's how much texture data was put through when using that setting.
Just remind him that yours has the latest GDDR3 memory (not that that makes a HUGE difference by any means, but it sounds fancy and convincing
but the 6800GS would definitely beat out a regular 6800.
It basically goes
The 6800GS is basically equivelant to a 6800GT, generally pretty dang close if not better in stock conditions, but once you overclock that thing, it can surpass the 6800Ultra which the 6800 regular simply can't do.
I know there were benches when the 6800GS was supposed to first come out and it was getting like 6500 in 3dmark05 when it OC'd to some pretty massive speeds.
Just tell him the 6800 is barely above a 6600GT and yours is closer to a 6800GT and if overclocked can be the same if not better than a 6800Ultra and mention the whole GDDR3 thing and whatnot as well.
I'm pretty sure that 6800 doesn't have GDDR3 anyway.....but if he's not computer savy you can just say that anyway
By the way I want 10% of the sale as commission for helping you out