Hi I also have a PCG-K23 and I am thinking of upgrading ram, but why can't intall 2 1gb ram sticks in it?

Because, by design, the PCG-K23 can only take a maximum of 1GB (2x512MB) of compatible RAM. If you install 2x1GB then either the memory will not work at all or the laptop will only "see" 1GB (2x512MB).

Because, by design, the PCG-K23 can only take a maximum of 1GB (2x512MB) of compatible RAM. If you install 2x1GB then either the memory will not work at all or the laptop will only "see" 1GB (2x512MB).
Ah, OK thanks will stick to 1gb max

Yes - but remember that means 2 x 512MB of compatible SO-DIMMs and not 1 x 1GB with the other slot left empty. Probably safest to use one of the online memory configurators (e.g. at Kingston.com, Crucial.com, PatriotMem.com etc.) to suggest guaranteed compatible parts rather than taking the chance with what looks like compatible parts as I did!