2.4 / 2 = 1.2

Each Radeon HD 4870 produces 1.2 TFLOPS, about 0.2 more than one Tesla GPU.

Two 4870 X2s equal 4.8 TFLOPS, 0.8 more than four Tesla GPUs.

I think the parent's point was that even when an HD Radeon 4870 X2 is made up of two cards they're still connected and recognized as one. Thus, with "fewer" cards and fewer slots you could achieve more performance. Or you could use the other two vacant slots for yet another two 4870s: Four of them in crossfire would equal 9.6 TFLOPS, 5.6 more than four Tesla GPUs.

Futhermore, I would assume two GPUs that are closely interconnected as a "single" card (4870 X2) would be better than a pair of GPUs connected through a combination of the motherboard (x2 Tesla GPU) and custom interconnects.

I'm not implying that an HD 4870 is a viable alternative to a Tesla GPU but the "performance" is more than just comparable. As it's been mentioned before, the hardware concerned is meant for precision and not speed, otherwise known as performance. Then again, you could compensate for in-accuracy by using all that computing power to make multiple passes rather than making sure your initial calculations were accurate.

Note: Emphasis by me in all quotes provided.

Not being too specific, the metals in the Earth's core create a magnetic field which shields against the solar wind, among other things. That same magnetic field has been getting weaker though...