This post in the AMD's Knowledge Base explains (737-24498) why nForce3 owners that are using an ATi AGP video card are left without 3D acceleration after upgrading to Vista:

Attempting to install the graphics driver on a system containing an NVidia NF3 chipset and an ATI AGP graphics adapter may result in the driver failing to install and the 3D engine not activating.

In Windows Vista 32bit or 64bit versions, the Device Manager will show a (!) mark and the AGP card will operate in PCI mode only. When the properties of the device are shown, the error code will show as code 43.

This issue has been seen with other AGP chipsets such as VIA and SiS but has since been resolved by chipset driver updates from these manufacturers.

AMD's AGP adapters require correctly functioning AGP support from the operating system in order to run the WDDM driver under Vista. This WDDM driver has no way of working around the absence of AGP support.

It seems that this problem seems to apply only to users running dual-core Athlon 64 X2 processors. nForce3 users with ATI cards and single-core processors may not be affected, although nForce3 chipsets are still officially unsupported in Vista.