Yep the FX-55 is more powerful than the 3.6 extreme. However the extreme's are good for overclocking and with some good cooling you don't have to get it up near 5 ghz for it to compete with the fx-55. About 4.3ghz or so is competitive with it, since the extremes are faster.
Yeah, i do see more Celerons than Semprons, but Semprons are basicly Athlon XP's rebadged, and Athlon XP's arnt that bad. And if some Semprons had mobile capability, they should have the wonderful overclocking ability of Mobile Athlon XP's.
The newer chips from AMD and Intel, the heat dissipation is quite different. The newer Intel Chips (3.4ghz Presscott) has 115 watts of power dissipation.
The newer AMD Chips (Clawhammer FX) has 89 watts of power dissipation. And the newer 90nm AMD's have a power dissipation of 55-60 watts.