In simple words your 32 Bit system won’t be able to use more than 2 Gb of RAM, even if you put 4 Gb in it . So 1Gb is just waste , as no other software will be able to use it neither your operating system . It actually has to do with the amount of memory addresses 32 Bits can generate .
But if you use 64 Bit operating system then it will be able to utilize all of your RAM . The link you posted earlier suggests that your system have 64 Bit OS . That is why I thought its good that you have 3 Gb of RAM and putting in 1 more Gb of RAM will make it even faster .