The others zero-in on your problem. Allow me to explain further or in basic terms:
Your PC may allow a total of 2gb of ram installed but only as 1gb per ram slot. You can't defeat this and there is no word-around. There is the outside possibility of accepting that new 2gb stick alone provided the bios is updated to the latest release and even do that as a last resort, no sure thing.
However, you maybe best served to return that new 2gb of ram and get 2x1gb ram set. IMHO, return and swap for true compatible ram sticks. Also, you'll need to remove the 512mb ram stick in order to install the new 2x1gb set.
So we finally got some RAM for our horribly under-RAMed eMachines W3623 running vista. It had 512mb and it was just unbearably slow. I bought a compatible 2GB RAM chip, installed it and the computer is still as slow and task manager only shows 512(510)mb of the 2GB. I've tried reseating the ram twice and upgrading the bios but I've had no luck. Anyone have any ideas? Thanks!