When comparing the prices, please remember that the 2008 mac pro's have 2 quad core xeons inside = 8 cores total, not just 1 quad core. The 2.8GHz quad Xeon processors cost almost 1000 euros alone! You can build a very good pc for less than 1000 euros if you choose intel core 2 quad processors and ddr2 memory etc. but if you build a pc using the same parts as the mac pro, the price will be about the same. If you buy a mac, it works when you turn it on. You can't really know whether a custom built 8 core pc will work without problems before you turn it on for the first time.
I just visited Dell's website and calculated a price for a Dell Precision T7400 workstation computer with similar specs as the mac pro, 2x quadcore intel xeon 2.83GHz, 4GB ram etc. The price was about 3700 euros! The mac pro is alot cheaper, even if you decide to order the extra memory from apple!
Apple have always priced their extra memory and hard drives high. You should always order the base model of the mac and purchase the extra ram etc. from elsewhere. That way the prices will be reasonable. I paid 2250 euros for the base mac pro 8-core model, installed my 2x 500GB samsung hard disks from my earlier mac and ordered 8 gigs of memory from OWC US which cost only about 300 euros. The total price for the mac was 2650 euros. A similar spec Dell Precision would cost over 4000 euros according to the Dell website. The mac is very quiet and fast. I can't hear almost any noise from the computer when it is located under my studio desk, a huge improvement from the G5. And the stability of the OSX...
Anyway, apple should sell a cheaper mac which could be upgraded as well. The iMac and mini are not very good in this regard and in case of a failure, you have to bring the whole mac to service. Anyway, in the case of the mac pro, you can switch any parts except the motherboard. I haven't had any hard drive failures/crashes with my macs in 4 years. And hard drives can always break down, it doesn't matter whether you're running a pc or a mac, they use same parts. Always remember to make backups, no matter what machine!
edit. Corrected the Dell prices, the default video card was too expensive.