I've upgraded my ram from 1gb to 1+2 gb. Both are 667mhz but still they don't work in pair but are fine individually.
p.s- ive an asus p5nmx mobo
Well, try resetting your CMOS and then update your BIOS to the latest version. Then try installing the new 2GB module along with the existing memory. Also first make sure the second slot is working fine. Let us know how it turns out!!
Update BIOS, 2GB stick in slot 1, other in 2, have no idea of the mobo or CPU but IF they will work, they will prob need voltage adjustments....odds are also good they just won't play together, this is why one shouldn't mix DRAM
With 1 or other stick in, enter bios and make sure bank interleave is off.
if it still wont post with both, just run the 2 gig stick and try to trade the 1 gig for an identical 2 gig that you have now. That will maximise the chances of running as a pair, but you might still have to mess about.