If you have a fairly nice gaming rig running Windows Vista, you'll want to check into a pair of patches that Redmond released this week. These patches reputedly were available exclusively through Microsoft customer service for a time, but they're now available for direct download.
The first patch, KB940105, tackles problems caused by graphics cards with large amounts of onboard memory, such as 512MB or more. As the documentation explains, a large portion of a 32-bit application's 2GB virtual address space can be occupied by a local copy of the contents of video memory. When this local copy of VRAM gets to be 512MB or more, the application may begin to run out of address space, potentially leading to performance problems or crashes. Vista's new driver model works around this problem by virtualizing and managing video memory for DirectX 10 applications.
Sounds like a good idea, but here's where things can get pathological.
If a game uses DX9 or otherwise keeps its own local copy of video memory, the local copy will occupy application address space in addition to Vista's copy of the virtualized address ranges. Hilarity, no doubt, ensues—especially with, say, a 1GB graphics card. The patch modifies Vista's behavior so that "only allocations that are created as 'lockable' consume space in the virtual address space of the application," according to Microsoft.
The next patch, KB936710, is amusingly titled "When a DirectX 10 application runs on a Windows Vista-based computer that has multiple graphics cards, the computer does not use the secondary graphics card." Such a thing might be a problem for a SLI or CrossFire rig, no? The problem only affects DirectX 10 applications, and it's very straightforward: "the operating system does not forward driver-render requests to the secondary GPU." Hence, the second graphics card may be no help whatsoever in DX10.
Obviously, most folks with decent gaming rigs will want to consider installing one or both of these hotfixes. Nvidia put it to us this way: "They address a number of stability and performance issues in both DirectX 9 and DirectX 10. Without the hotfixes, you may experience crashing or performance drop offs. Many applications are affected, including BioShock, Company of Heroes, Call of Juarez, Battlefield 2142, Half-Life 2 and others."
Before installing them, you'll probably want to install Microsoft's two other major Vista patches intended to correct performance and stability problems, as well.