If you want a performance upgrade, try installing a solid state drive in place of either the super drive or the HDD. I did this to my 32 bit Intel iMac because it was otherwise at end of life since 10.6.8 is the last release that will run on it. I did this a few years ago already and I'm still not tempted to buy another new machine, although I'm typing this on my recent MB Air supplied by my company. I mostly did it because the HDD was dead, but as things would go, the hard drive decided to come to life the day I went to open it up. I figured I should try to turn it on one last time, and sure enough. For a month before that all I would get is a clicking sound.
It isn't an easy swap as you have to open it up, but I did it without any previous experience and it booted up and restored itself from the Time Machine backup first time. I just needed to have an original Snow Leopard disk.
After I had it fully set up again, I did the test of selecting ALL of my apps and clicking them open simultaneously. Only iMovie lagged behind the rest opened instantly.
I'm going to do a SSD update to my kids' MBPs (3-4 years old now) before they go to school in the fall. It is a lot cheaper than buying 3 new machines for them.
