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2Gb to 4Gb will be noticable depending on what you do, such as Photoshop, MSN and Music etc.OS X is 64 Bit so it will easily recognise 4Gb
youl be able to run a few more apps at the same time and games will be a little quicker. the only way its going to be noticeable is if your upgrading from 512mb to 2gb
just make sure the ram is compatible with your comp before you buy it. it would suck to get it and find out its the wrong kind.
It also really depends whether its ddr2 800 or 1200. If you have 800, and your motherboard is compatible with 1200, go for the 2 gigs of 1200 , it will be faster than having 4 gigs of 800