Chris Foresman for Ars Technica:

Looking at the graphics hardware differences between Mac models that are currently supported in Mountain Lion, at least one clear pattern emerges: older Intel integrated graphics aren’t going to be part of OS X’s future going forward. MacBook, MacBook Air, and Mac mini models with integrated Intel GMA 950 or X3100 seem to be out. Newer models with NVIDIA 9400M, NVIDIA 320M, or Intel HD3000 appear ready for the new hotness.The division for the iMac also appears to be pretty straightforward. The earliest Core 2 Duo models, which had a white polycarbonate enclosure, featured discrete GPUs with a 128MB of VRAM. The later aluminum enclosure iMacs, which are supported under Mountain Lion’s developer preview, include GPUs with 256MB or more of VRAM.

It’s worth noting that Apple has not yet announced final system requirements for Mountain Lion, so this information is subject to change. Regardless, if you have a machine that falls into the category Chris Foresman describes, it’s probably time to start thinking about a complete system upgrade if you plan to upgrade to Mountain Lion when it becomes available later this year.