You will waste money as your 32bit system can only access 3gb of RAM the rest is set aside.
However your motherboard may only allow a maximum RAM, check Motherboard specs.
3gb is IMHO better than 2gb. You may also have to use matched pairs eg 2 x DDR2 PC2-5300 if not the ram will only work at the speed of the lowest stick, not the other way round.
I have memory in use screen app and before with 2 gb it frequently showed 1800mb in use now with 3gb it shows a maximum of 2300mb in use and system is significantly quicker at opening applcations.
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How many?
is there a posibility to put 4gb in one unit?