I'd call BS on someone claiming 600+fps without minimum quality settings and resolution. Without real, standardized benchmarking settings and map conditions, I wouldn't trust that hardware performance table for squat.
Java is capable of dealing with multiple cores, threads, and high-performance graphics cards, but Minecraft isn't necessarily. If it's not coded properly to take advantage of the language and hardware, then performance will suffer. It's gotten better from 1.2.5 to 1.4.6, but it's still got a ways to go. Users with super-high performance hardware shouldn't automatically expect higher fps by having a higher card than every other card, but the performance on the medium and low end machines is lacking.