PC only reading 2 of my 3 GB RAM?

 I previously had 2x 512%26#39;s RAM in my PC, but placed 2x 1GBs in just now, so I should have 3 GB%26#39;s RAM, but on my system information it only says 2 GB? Why is this? :SThanks. PC only reading 2 of my 3 GB RAM?
what os r u on.PC only reading 2 of my 3 GB RAM?
[QUOTE=''r3351925'']what os r u on.[/QUOTE]Sorry for the delay, Windows XP 
Are you sure that 1GB RAMs were working?Change the slots.
what type of ram is it if its DDR2 there%26#39;s no point in having three Gigs as it wont run in dual channel you might as well just use the 2gb
[QUOTE=''adam0926'']what type of ram is it if its DDR2 there%26#39;s no point in having three Gigs as it wont run in dual channel you might as well just use the 2gb [/QUOTE]2*512MB %26 2*1GB so it will run Dual Channel 
to to run in dual channel it has to be ether 2x512mb 2x1gb or 4x2gb.
[QUOTE=''adam0926'']to to run in dual channel it has to be ether 2x512mb 2x1gb or 4x2gb.[/QUOTE]With that also runing Dual Channel.2*512MB %26 2*1GB also working Dual Channel 
[QUOTE=''adam0926'']to to run in dual channel it has to be ether 2x512mb 2x1gb or 4x2gb.[/QUOTE]
you wanna rephrase that? or did you mean to insinuate 8 gigs?
try running it with just the two GB sticks and see if it reads them.
[QUOTE=''adam0926'']to to run in dual channel it has to be ether 2x512mb 2x1gb or 4x2gb.[/QUOTE] No it doesn%26#39;t. It just has to have 1 or 2 pairs of matching memory.
What are your full ram specifications?What is your motherboard?