To prove my point about Halo 2's minimum requirements being driven by Vista ...
Doom 3 Minimum requirements:
http://www.doom4portal.com/doom-3-system-requirements
P4 1.5Ghz CPU (or equivalent).
384MB of RAM.
64MB graphics card (see below for chip details).
2GB of free hard drive space.
Left PC - Right XBox
Half Life 2 minimum requirements:
1.2GHz processor
256 MB of RAM
DirectX 7 compatible graphics card