I need help with dell dimension c521 ram issue.?

ok i have a 6400 ddr 2 gig stick i also have a 6400 ddr 1 gig stick and i have 2 5400 ddr 2 512 sticks now i have my computer standing up and from the bottom i have 2 black and on top 2 white slots. the slots do work cause i had on bottom work in up is 2gig black slot above that is 1 gig black slot above that is 512 white slot and above that is another 512 white slot. i went to crucial site and download in memory scanner and its not reading 1 of the 512s its saying i only got 3.50gigs im trying to max it to 4 gigs now i tried switching it to pair the 512s the following being bottom to top is bottom black 2gig and above that 512 black slot above that 1 gig white slot and above that 512 white slot and its still reading only 3.50 theirs no numbers on the slots except the only way i described them i hope someone can answer this question i know they all work cause before i put the 2 gig in i was reading 2 gigs with the 1 gig stick and 2 512 sticks and i had the 2 512s in the top 2 whites ones and the 1 gig on the one below that in the black slot and the very bottom black one was empty if anyone can help me thanks :)