Are there any known problems with populating all 4 dimms sockets ?

i know it doesnt show above 3gb properly


i have 2 x 1gb geil 800 mhz dimms and wanna put 2 x 512mb of the same type of ram in for a total of 3gb

dont wanna mess up a successful overclock of 400 fsb cpu at 3.2ghz memory at 800
and 1 other question im running 400fsb no probs why do people always suggest try 401 ? is there something special that happens with 1mhz more ?