PC4000 should work fine at 500MHz or any speed under it. So running it at PC3200 speed (400MHz) shouldnt be a problem. It also will actually help if you are increasing FSB - as it will work at up to 250MHz DDR (500MHz) allowing some headroom for overclocking.
I orderd by mistake corsair twinx 1024-4000 ddr-dimm 1024 kit w/two matched cm512-4000dimm, I meant to order pc 3200 but it doesnt seam to matter. I'll be using it in my athlon fx 53 with my x800pro card it should be built by thursday.