30% faster is just enough to stay ahead of R680 (as it has been rumored to be about 15% faster than a 8800 Ultra).
So this would make a "9800 GX2" roughly 15% faster than a HD3870x2/R680.
Depends on the clocks of GPU/Mem and mostly in the segment of price.
Rumoured R680 says 350€ witch is a very agressive price.