Cleeve, I see clearly where you are coming from, but I have been reading many benchmarks involving both of these graphics cards. To be blunt it is difficult to distinguish the two, different websites will offer different results even on the same benchmarks.
Plus you yourself Cleeve could be subject to "fanboyism" as you put it, as you have a Radeon X1800XL, a shade poorer than the 1800xt, which our respective OP is planning on buying.
what would be the advantage of the 512 ATi over the 256 ATi?