Spanky Deluxe
macrumors 603
The things you can upgrade include:
Storage: 5 internal bays on a Mac Pro (4HD + 1Optical). You would need an external enclosure like this for the equivalent in an iMac. With a MP you can also add an eSATA card for additional external storage. Finally, you can swap the optical drive or add a second one if you like on the MP. While it might be possible to update the iMac slot loading drive down the road, it may not be practical.
Memory: 4 DIMM slots in both the MP and iMac with up to 16GB capacity.
Graphics: Graphics card choices are limited for both a MP and an iMac. You have the GT120, ATI 4870, and GTX285 for the MP and can add multiple cards if you want to run more than 2 monitors. I believe you have just the 4850 for the top iMac but can drive two displays from the included card.
Processor: The processor can definitely be upgraded in the MP. It's highly likely it will also be upgradable in the iMac (although with a bit more effort). However, the processor in the iMac is pretty damn good.
Display: Of course with the MP, you can change displays on a whim... not the case with the iMac... however, you are basically getting the display for free with the iMac.
Yeah the Mac Pros definitely have a lot more upgrade potential than the iMacs. However, just because its there doesn't mean that you'll use it. I've upgraded my Mac Pro a fair chunk over the years. I've added more RAM, added many hard drives (6.25TB to be precise) and upgraded the graphics cards twice. However, those hard drives could have been externals. RAM wise the Quad costs about £850 in the UK to upgrade third party to 16GB compared to £920 for the iMac so there's not much in that and the difference will be even smaller in two year's time.
So the biggest difference in terms of upgradability between a Mac Pro Quad and an iMac is the display and the graphics card. However, considering an iMac 27" i5 costs £1599 and a Mac Pro Quad with a 24" Apple Display is £2533.99, you could quite easily sell the iMac after a year and a half and buy a brand new model with a better display and better graphics and end up spending the same or less over a 3 year period.
Of course, this will all be a little simpler when the Mac Pros get updated.