I have two machines at home:
P4 2.8Ghz, 0.75Gb RAM, 512Mb AGP Graphics card, Windows XP
AMD X2 2200, 4Gb Ram (3 usable), 512Mb PCIe Graphics card, Windows Vista.
The machines are networked, and the XP machine is my machine of choicce as it's faster at everything. Whatever they did with Vista kills whatever advantage the new machine specs added. I turned everything off with Vista as well. Want to install XP on it but have decided to wait until windows 7 is out then just upgrade.
Apparently upgrading Vista is purchasing the upgrade disc and it installs over Vista. The XP upgrade involves a new install.
(source PCMag.com,
The Windows 7 Versions: What You Need to Know - News and Analysis by PC Magazine)