I'm back now, and I want to say thanks to everyone who posted in this thread - I've learned an enormous amount from this thread, even or especially when people disagreed with each other.
Thanks to evilsharpie for posting so much (!), and to ViRGE for linking his pics as evilsharpie's large pics made it impossible for me to read this thread for a while. The S3 Virge was probably my first ever PC graphics card, so bit of history there.
My MCITP books have arrived now, and I've started on the first one, 70-680, Configuring Windows 7. I'm finding it unexpectedly challenging already as it jumps straight into system images and deploying them over networks, and BitLocker and BranchCache.
I'm used to working with DNS and Active Directory as I work with them every week, but these things are new to me.
Regarding hardware, the 70-680 Win 7 book says it only needs 2 VMs, and 4GB, which I can deal with easily on my macbook.
Out of all the MCITP Enterprise Administrator modules, the one with the highest requirements is the next one, 70-640, Configuring Windows Server 2008 Active Directory, which says it needs 7 active VMs and 8GB of RAM.
All the other modules say they are happy with 3 VMs or so, and around 4GB of RAM.
I think I'm gonna wait and complete 70-680 Win 7 and make a start on 70-640 Active Directory before I buy any more hardware. If I'm lucky, I should be able to get away with a m-ATX board and an i7-860 with 8GB.
Of course, I'm always suspicious of MS's 'minimum RAM requirements' which is why I'll start the module first and see how that goes. There's no point splashing out on 12GB if I only ever use that for a couple of optional labs.
To answer a couple other points in this thread: I work as a very inexperienced server admin, 2 days a week, for a charity with 50 staff and an AD structure, who are paying for my exams and books. This computer I will be paying for myself. I'd like to use it for other things apart from MCITP labs.
Someone suggested I buy some ancient Dell rackmount servers. Nice idea, but the RAM they needed was the killer. I also looked at getting a Core2Quad, which comes about £180 cheaper for mobo and chip. The DDR2 it takes costs exactly the same as DDR3.
As I'm spending my own money, I'd rather pay a bit more for something that has legs going forwards and not spend a similar amount of my money on something that is more or less at the end of its retail life.
Cheers all x RT