if you don't mind running at stock 400 mhz (200mhz X2 from dbl pumped fsb) then keep it at that, but since you're using hyperx check your manual for bios settings. I know athlon xps are different, but in my a7n8x-e deluxe bios. if I want to enable "spread spectrum" for memory I can only do it at 100,133, and 200mhz, when the resulting freq. is 266, 400, and so on; the stock settings for pc2100,2700, and 3200. check to see if it's on/enabled, if it is it's possible they're conflicting, hence boot probs. my bios settings for my boards have always had spread spectrum as default. check it out. hope it helps.