I currently have 2X1Gb Mushkin memory and I was wondering if there are any problems with installing another 2X1Gb?

2X2Gb seems the preferred option so what are the advantages and disadvantages between the two configurations?

This memory is no longer available but I`ve found somewhere that still has some stock left so I`m thinking of buying another 2X1Gb kit for future proofing incase I go to a 64bit OS.