I bought the original Intel based Mac Pro back in January of 2007. It had two dual-core Xeon 2.66 Ghz processors (four processors total). I think it came with 3GB of memory and I added an additional 4GBs for a total of 7GB. About a year ago I added the ATI Radeon HD 4870 graphics card to my system. The Geekbench scores for this system were around 5000, respectable, but many of the iMac and MacBook Pros that are being sold have Geekbench scores over 10,000.
I am a photographer so most of my heavy lifting is done with tools like Adobe Lightroom, Apple Aperture, and Photoshop CS5. I use CS5 plugins like Nik Color Efex Pro and Portrait Professional. Most of these tools take advantage of the GPU so the ATI Radeon HD 4870 gave me a good boost. But there were times with Aperture, exporting from Lightroom, and running some of the CS5 plugins that my system got a little sluggish.
So last summer I had to decide to buy a new Mac or upgrade my 2006 Mac Pro. The future for Macs is definitely going towards buying high powered laptops with an SSD (solid state drive) and using Thunderbolt devices for access to your high speed drives and monitor. The problem in the summer of 2011 was that there were few Thunderbolt peripherals available, and you knew that the current, heavy MacBook Pros were going to go the way of the MacBook Airs. I really use the internal drives of my Mac Pro and didn't want to give them up, and someday Thunderbolt drives will be my answer. So I looked into the current Mac Pros, they are fast, but they currently don't support Thunderbolt.
Given that we are in a period of transition with Apple Mac products, I decided to extend the life of my current 2006 vintage Mac Pro.
I replaced my original 256GB main hard drive with an 256GB SSD (Crucial Technology $372). I added 4GB ($108) to my memory taking me up to 11GB. And I swapped my two dual-core Xeon processors for two quad-core Xeon processors ($200 on ebay), giving me an eight core machine.
My Geekbench score went from 5000 to over 10,000. I spent $680 and I got a machine that was current in terms of performance vs. spending what would have been over $3,000 to get a new Mac with an SSD card and 11GB of memory. The new machines have faster cores, so applications that only run on a single core aren't as fast on my machine as they would be on a new Sandy Bridge Intel CPU. But for most things my performance is much better.
After I did this upgrade and things were working well, I got greedy. I decided to add a USB 3.0 card to my Mac Pro last November. The only card I could find that claimed to work on a Mac Pro the was CalDigit SuperSpeed PCI Express Card. My system has crashed more times in the last two months than in the prior five years. I suspected the CalDigit card, so I removed it two weeks ago. Guess what ... no crashes since. You have to install a USB 3.0 driver from CalDigit to make this card work because the operating system, Mac OS X Lion, doesn't support USB 3.0. It seems to be that this driver doesn't play well and causes the system to crash.
CONCLUSION: For $680 I have extended the life of my Mac Pro for probably at least two years ... well worth the money.