The good news: OS X 10.8 (Mountain Lion) has gone gold master and will probably arrive this month.

The bad news: It might not run on your older 64-bit Intel-based Mac.

According to Ars Technica, Apple has updated the upgrade page of its website, tweaking which models will be supported by the latest big cat.

Despite a few older Macs being otherwise 64-bit capable — a requirement for both the current OS X Lion and the upcoming OS X Mountain Lion — it appears that 32-bit graphics drivers may be to blame for Apple’s decision.

“While Mountain Lion is compatible with any Mac capable of running a 64-bit kernel, the kernel no longer supports loading 32-bit kernel extensions (KEXTs),” Ars Technica explains. “Since those older drivers are 32-bit, Mountain Lion won’t load them. We believe Apple decided it was better to draw the line in the sand for some older machines rather than invest the resources into updating the drivers for these older GPUs.”

As noted by Apple’s OS X Mountain Lion upgrade page, supported Macs now include the following:

- iMac (Mid 2007 or newer)

- MacBook (Late 2008 Aluminum, or Early 2009 or newer)

- MacBook Pro (Mid/Late 2007 or newer)

- MacBook Air (Late 2008 or newer)

- Mac mini (Early 2009 or newer)

- Mac Pro (Early 2008 or newer)

- Xserve (Early 2009)

If you’re running close to edge of this cutoff, now might be a good time to consider a new Mac anyway, as Apple will chuck in a free copy of Mountain Lion with the purchase.

Stay tuned for additional details as they become available.