This is a
verified "known issue" only to the extent that when run under OS 10.6.4 certain specific NVIDIA cards do not perform well with applications that use NVIDIA's CUDA architecture. This requires using a CUDA driver that comes from NVIDIA, not Apple. (It is not provided with the OS.)
Moreover, only certain cards are supported by the currently available driver. See
http://www.nvidia.com/object/macosx-cuda-3.1-driver.html for more about this, particularly in the "Supported Products" tab.
Beyond that, Apple provides standard graphics/video drivers only for GPU's it provides as OEM parts (including built-to-order options). Aftermarket cards, including those designated for use with Macs, may require different drivers developed & provided by the card's maker. For aftermarket cards not specifically designated for use with Macs, no suitable product-specific drivers for OS X may have been developed. These cards may work with the Apple provided drivers but since they were not designed for them, performance may (& probably will) be less than the card is capable of providing.
This appears to be the case for the Geforce 250 GTS: when I search for drivers for it at
http://www.nvidia.com/Download/Find.aspx there are zero results for OS X drivers for it, although there are plenty for Windows.