Macs that were released new as of July 20, 2011, will generally not run Snow Leopard. Other than that, the following statements are true:
All Mac Pros will work with Snow Leopard (10.6.x), and they look like:
The PowerMac G5 towers which look like:
All Apple notebooks labelled MacBook with at least 1 GB of RAM, MacBook Pro, and MacBook Air below the screen will work with Snow Leopard.
Apple notebooks labeled iBook, and Powerbook beneath the screen will not work with Snow Leopard.
Will work with Snow Leopard.
If they are iMac Intel they can upgraded to Snow Leopard. To tell if they are Intel, they will have an EMC# on the base which is enumerated 2104, 2105, 2110, 2114, 2118, 2111, 2133, or 2134.
Otherwise they are iMac G5, and can't be upgraded to Snow Leopard.
You can also tell if it is an iMac Intel by selecting Apple menu -> About This Mac. Core Duo and Core2Duo are Intel, whereas the G5 are not.
Notes: G5 refers to the CPU made by IBM for Apple before the migration to Intel CPU in 2006. It was found on iMacs, and PowerMacs. Powerbooks and iBooks maxed out using the Motorola G4 CPU, only to be replaced by MacBook Pros and MacBooks in 2006.. Intel made the CPU found in 2006 and newer Macs, and these are referred to as CoreSolo, CoreDuo, Core2Duo, i5, i7, and Xeon. Don't confuse a G5 for an Intel CPU Mac. They are not the same except in exterior design when it comes to the iMac, and the means to tell them apart is stated above. In 2006, the Mac Mini changed from G4 to Intel CoreSolo CPU. In 2006 the iMac changed from G5 to Intel CoreDuo CPU.
All Intel Macs with sufficient RAM older than March 19, 2010 can take the retail 10.6.3 installer disc. All Intel Macs with sufficient RAM older than August 28, 2009 can take the 10.6.0 retail installer disc. This disc must look like and can't say Upgrade, Dropin, or OEM on it.
It is recommended those upgrading from PowerPC follow this tip:
It is recommended you backup your data at least twice before upgrading any software.
It is recommended you check these listings for compatible 10.6 software from:
C!Net, Snow Leopard Wiki,Macintouch, and Apple's listing of compatible printers and scanners
Other User tips of interest in terms of upgrading, include those on upgrading to Lion 10.7:
And Leopard 10.5 (though I would not downgrade to Leopard without erasiing your data first):