I think you have confused the XScale with the StrongARM... The XScale has very good FP poerformance, MP3 and DivX seem to run fine on the 400Mhz version. The Xscale even has some DSP instructions in there :-)
My point is, that if you wanted to make a new Amiga you would have to sell it on the original strengths of the amiga brand, Cheap, small and versitile... not something which a PPC board with standard PC components is able to provide.