Just purchased an 2011 MacBook Pro (Quad i7) to replace my 2008 MBP, which had a dead logic board.
To be honest, the 2008 machine was still doing quite nicely with 6GB RAM and a fast FW800 drive to hold my LR catalog and images. That said, the difference out of the box with the 2011 machine was enormous; and then I put in 8GB of RAM and a 240GB SATA-III SSD, and now the speed is insane. This machine flies through 5D2 RAW files.
In general, I would normally upgrade about every three years. Back when I was dealing with 8MP 1D2 RAWs exclusively, my 2008 machine was perfectly fine. With 21MP 5D2 RAWs (not to mention 400MB film scan TIFFs), the 2008 machine was sorely lacking.
A RAM upgrade and an SSD can certainly breathe new life into any machine.