RAM problems

Im trying to upgrade my RAM from two 128mb cards and two 64mb cards to three 128mb cards and one 64mb card. basically im replacing a 64mb card with a 128mb card. the cards are arranged in the following order:

1.128mb
2.128mb
3.64mb
4.64mb

im trying to upgrade to this:

1.128mb
2.128mb
3.128mb
4.64mb

however everytime that i replace one of the 64mb cards, i get a beep code from my computer. i have ruled out bad RAM alread y and i have come to the conclusion that in any other arrangement than

1.128mb
2.128mb
3.64mb
4.64mb

i get a beep code. regardless of whether or not i use the new RAM. how can i upgrade my RAM?