I have a VISTA 32-Bit OS & AMD athlon 64x2 with 1024mb RAM (2x 512mb) DDR2 SDRAM (PC4200)..supports up to 4GB of ram with 2 banks (2 pairs at banks)..im planning to add more Memory probably a 2GB (2x 1GB) same speed PC2400 but greater module.. correct me if im wrong here about my idea that the speed only matters not the size..512MB & 2GB will run smooth as long as they are identical in speed PC2400..planning to put the 2GB(2X1GB) in 1 banks & may old 1024MB (2x512MB) on the second bank to make it pair.. is this a ggod idea.. enlighten me with some idea about or correct me or a better advice if possible.. thanks!! have a nice day to everyone..

Yes, in general mixing sizes is ok, as long as pairs of modules are same sized. Having different sizes in the two banks may have an impact on dual channel usage, I can't speak to that particular aspect.
Val