Windows vista genearlly LOWERS fps, not only is it a more taxing OS, but also supports DX10 features, which will be harder on your graphics card compared to a DX9 version of the same scene.
Vista is know to take about one gigabyte of RAM for background applications, while XP SP2/3 takes less than half that much (if my memory is right, if its wrong just know it takes significantly less RAM), so you will also get a performance boost from that, though not necassarily in your FPS, RAM doesn't always participate in graphics rendering.
32 bit OS's can only handel 4 gigs, and its generally accepted that if you have 4 gigs, you will only have the power to use about 3.5 gigabytes, but with DDR2 RAM so cheap, its probably worth it to buy 4 gigabytes.
I am not sure if WOW uses DX10 graphics, but your performance will almost certainly be higher on XP, BTW xp - 64 sux, vista 64 sux but not really worse than vista 32
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