Like OS X Lion, when Mountain Lion launches later this month it will be sold via the Mac App Store. However, unlike Lion, which would download even if your Mac didn't have the spec to support it, it seems that the Mac App Store will assess your Mac's suitability for the new operating system before it lets you download, and pay, for it, according to 512 Pixels.
Mountain Lion has the following specs. It requires OS X 10.6.8 or later, 2GB RAM and 8GB available space.
The following Macs are supported: iMac (from mid 2007 model); MacBook (from late 2008); MacBook Pro (from late 2007); Xserve (from early 2009); MacBook Air (from late 2008); Mac mini (from early 2009) and Mac Pro (from early 2008).