With the commits that landed hours ago in the Nouveau DRM repository, Z compression is now enabled for NV30 and NV40 class NVIDIA graphics processors.
NV20 class hardware already had Z compression support since 2010 but now NV30 and NV40 class hardware has enabled Z compression support. Z compression is the compressing the Z-buffer image depth coordinates to reduce bandwidth on Z reads and writes. Z compression is one of the parts on the AMD side referred to as HyperZ. Both NVIDIA and AMD hardware has supported Z compression hardware for quite some time.
The NV30 class hardware is the GeForce 5/FX series while the NV40 class hardware is the GeForce 6 and GeForce 7 series. The Z compression support may lead to performance improvements for this hardware, but we will have to wait until new Phoronix benchmarks are conducted.