I currently have a Asus P5W DH Deluxe motherboard with Corsair XMS2-5400 TWIN2X2048 memory (2GB).
Now I'd like to expand and get some more memory, however, I can't help to wonder if it is OK to mix types.
Example: Would it be ok for me to buy Corsair TWIN2X 6400 DDR2, 4096MB CL5, so I get a total of 6 GB? Also, the specs are slightly different, (PC5400 vs PC6400). I also read somewhere that you need to have an OS that is 64-bit in order to run RAM over 4 GB, is this true?
If you mix different speeds, it should all run at the lowest of both.
Moreover, since all modules are not of the same size, your board might revert back to single-channel (instead of dual). If placed correcly (3GB per channel), some boards will be able to run in dual channel.
Finally, yes you need a 64 bits OS to use more than 4GB.