I just put 6 GB of RAM in my early 2008 MacBook Pro and am experiencing a significant performance boost. The Apple website says my Mac should hold a maximum of 4 GB of RAM.
The image above shows that my Mac is now running with 6 GB of RAM. Previously I was running two 2 GB sticks. I just removed one of them and installed this 4 GB stick made by Crucial, for a new total of six.
My Adobe Lightroom 4 catalog now opens at least twice as fast as previously. The catalog is approximately 700 MB in size and has a little over 48,000 images in it. The previews file is a little over 15 GB. This app is the main reason I wanted to stuff more than 4 GB of RAM into this Mac. It was a bit sluggish before. Now it opens briskly and browses through images much more smoothly.
So, if you are stuck with an old MacBook Pro, like me, there are a few things you can do to speed it up. You can start by giving your hard drive a check up and repair through Apple's disk utility app. If you are using the original 5400 RPM drive, you can replace it with a 7200 RPM drive. You can even defrag your hard drive if you want. I used iDefrag, and you can read about it here. Then put 6 GB of RAM into your Mac. And don't forget to backup your drive each step of the way so you can recover if something goes wrong.
I'll report back if I run into any problems.