A MacPro is expandable…
I found a first generation Mac Pro on Craigslist, with a correct price-tag, that make me think twice about using my quad-core PC or using the “real thing” instead. 1500$ for an expandable computer that might least for 2 or 3 years is not too much for a developer.
Then, I checked graphic card, because I would like to have at least GeForce 8800 or equivalent (GT 130) for CUDA & OpenCL development, and maybe a Radeon 4870 for OpenCL and games too 
Naturally I needed to install Windows Vista 64bits, or Windows 7 64bits, planning to install 8GB at first and probably 16GB later on this MacPro.
Las… Apple doesn’t offer serious graphic card for this generation, they are all dubbed for Early 2009 Mac Pro, or at least 2008 Mac Pro, and this one isn’t compatible with 64bits Windows!
Prefer to forget it, a GeForce 7300GT is on a par with IGP 9400M, and having 3GB on Windows on a 16GB computer seems totally stupid!
Mac Pro might be expandable, as PowerMac were, but the real problem is Apple doesn’t offer any expansion for 3 years old computer, so it’s almost as closed as an iMac, if you don’t buy expansion cards when brand new!
[UPDATE 21/01/2010] Apple proves me that I was right choosing to keep my quad-core PC instead a first-generation Mac Pro: the Mac Pro, 64bits quad-core with Xeon processors, is *NOT* compatible with Vista 64bits or Windows 7 64-bits, and Apple ceased to support it for BootCamp (windows installation assistant and drivers). A 2-years old Mac Pro 4×2.66Ghz is officially unable to launch Windows 7 64bits, contrary to my 5 years-old PC!!! So stupid!
