Vista - Resolved: Memory errors in Guest machines after adding extra 4GB RAM to Host

Thanks everyone for your advice and input.
I bought another pair of DIMMs from the same manufacturing batch as the
second pair ("Elixir" brand.)
Everything works fine now.

I think this is noteworthy for anyone building a Hyper-V system:
Make sure you have *exactly* matched memory.
There was absolutely nothing "wrong" with the RAM I swapped out.
It passed multiple "Extended" checks with the Microsoft Memory Checking Tool
(and is now happily humming along in my workstation...)
Also make sure that your processor supports virtualization (it will say so
on the box, if it doesn't...then it doesn't)

All things said and done, I can attest that it is possible to build a very
workable study/test Hyper-V server for about US$500.
My rig comfortably runs Server 2008 Enterprise as the host + 3 x Server 2008
Enterprise + 3 x Server 2008 Core Enterprise + 2 x Vista workstations
concurrently as guests.
Core Server requires very little RAM, you can install and run it with just
384MB.
Full Server works well with 768MB unless you are running Exchange, SQL or
suchlike (increase memory as you need it.)
Vista running Office 2007 needs 1GB+

1 x Asus Intel P43 Motherboard (P5QL-E)
1 x Intel E8400 Core Duo 3.0GHz CPU
4 x Elixir 2GB DDR2 800MHz Memory DIMMs
1 x 160GB SATA hard drive (for host system, ISO images, install files and
..vxd templates)
1 x 500GB SATA hard drive (for guest machines)

Having 2 hard drives is a must, even running 1 guest on the same drive as
the host system slows things down immeasurably.
If you have a bit more cash to spend then go for more hard drives to spread
your guests over.

One last note - most BIOSes have a feature to re-map the PCI address space
away from 4GB to 32GB to allow for 64bit O/Ses.
Do NOT disable this unless you want to make your server crawl like snail!