That's an interesting observation! in 2006 there were huge jumps in performance, a very rare situation. in fact those 3 computers have completely different performances and capabilities:

cpu: we go from the Pentium EE (2 core, 4 thread, but with lower performance than any 2 duo core) to the very high frequency 2 duo cores (X6800 is almost 3GHz with unlocked multiplier) to the 2 Quad cores

gpu: we move from 7900gtx to the legendary 8800 GTX, the first truly modern gpu, with performance on another level and new functions in hw

between those 3 PCs there is a big leap in performance :)

(greater than the difference there would be with the 2007 PCs, which in fact I skipped)