Want to point out that on the linkmentioned above that Apple now official supports 8gb for all late2008 models that the 15" 2.4ghz unibody model is not included. It was the low end model of the first release of unibodies, which I think was only released in 15", 17" came a little later (maybe the alum mb came at the same time).
Regardless, apple still only supports 4gb in that model. But I believe this is the model that poster (fb48love, sorry I'm writing this on my iPhone and can't remember your handle) says he got to work. I would love to bear more, like what happens, when you stress usuage above 7gb.
Btw. I have a 13" 2.53ghz mbp with integrated graphics chip only, 4gb ram, in activity monitor it shows 3.75. I think this is normal. I think 250 is set aside for graphics, effectively reducing your memory by 1/4gb.