there is a difference between seeing a stick and running a stick.
a friend was running a computer with win98, then he upgraded to XP and it ran real slow with 128mb RAM (DUH) so he tookit back to the store he bought it from and they added a 512MB stick for 784 total. It still ran slow - for a year, then it was almost unusable, so he asked me to look.
I installed SiSoft Sandra and what do you know, the mobo only supports 512 MAX so the second 512 stick that he paid for was not even being used. RAM showed 784, but only 128 used. This was seen when I did a memtest86 run from a floppy and only saw 128 being accessed.
You made no mention of mobo, but apparently you cant run SPD in BIOS (serial presence detect) - must set ram timings manually, and you ma be having trouble with 2 way or 4 way interleave, especially with mixed single and dual sided sticks. Or the DRAM's may be different density.
So your next step is to load up Sandra and do their memtest and see what it says.