Anything more than 3GBs is a waste of money with 32bit OS as large sections of the 3-4gb section of system memory are reserved for things called memory-mapped devices...this scarily includes the 128MB-1GB graphics card u may have.
Even if your using a 64bit OS no difference there. U will still only get about 3gbs depending on your m-m-ds. but the thing with 64bit is that it allows you to go further than 4gbs of system mem all the way up to 8gbs. but you will only see about 7gbs of usable system mem because of the same principle.