Hand-in-hand with the launch of the GeForce 9800 GTX, Nvidia has released a new set of ForceWare drivers. This 174.74 release brings support for Nvidia's latest high-end offering, and it also support GeForce 8-series integrated graphics chipsets and other GeForce 9-series cards. However, users of other Nvidia graphics chips will have to make do with older ForceWare releases for now.
High on the feature list for the 174.74 release is Quad SLI support, which should allow folks to pair up two GeForce 9800 GX2 graphics cards in a single, quad-GPU configuration. The new drivers let users run three GeForce 9800 GTX cards in tandem in a single PC, too. For everybody else, Nvidia says the 174.74 drivers bring improved performance in "many" DirectX 9, DirectX 10, and OpenGL applications, not to mention support for "GeForce 3D Stereo Technology." According to this page the 3D Stereo feature applies to both specially designed monitors like Zalman's ZM-M220W and ZM-M190 as well as "anaglyph displays" with red/blue stereo glasses.
ForceWare 174.74 drivers are available for Windows Vista 32-bit, Windows Vista 64-bit, Windows XP 32-bit, and Windows XP 64-bit. However, Nvidia points out that the Windows XP versions don't support three- or four-way SLI setups, and that the 3D stereo feature is only available in 32-bit versions of Vista. Release notes are available in PDF format here for Vista and here for Windows XP.