Your old MacBook Pro stuck on 4GB? Perhaps not...
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However, researching on the internet says it is possible to upgrade to 6 GB. (Very useful when it is getting tight running applications in 4 GB).
I've just upgraded a MacBook Pro (early 2008) (Penryn processor @ 2.4 GHz) to 6 GB. Speed difference is not noticeably different and stability (so far) is as good as ever.
I got a RAM stick from Mr Memory.
1 X 4Gb 200 Pin SO DIMM DDR2 PC2-5300 667MHz @ GBP 53.35.
So far, so good.