Confused. About ram

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Ive been think ing about upgrading the ram on my late 2008 mbp. It's the first unibody model. Lately it's been seeming very sluggish. What is the max ram it can take? It's a 15" mbp with 2gb now. I've seen online that it can handle either 4 or 8gb. But not too sure. I don't want to buy 8gb and it not fit.
Thanks