Yes, there's been no updates to 10.5.8 for quite awhile, next is paid Upgrades.
Snow Leopard/10.6.x Requirements...
General requirements
* Mac computer with an Intel processor
* 1GB of memory (I say 4GB at least)
* 5GB of available disk space (I say 30GB at least)
* DVD drive for installation
* Some features require a compatible Internet service provider; fees may apply.
* Some features require Apple’s MobileMe service; fees and terms apply.
Which apps work with Mac OS X 10.6?...
http://snowleopard.wikidot.com/
It looks like they might still have it...
http://store.apple.com/us/product/MC573Z/A?fnode=MTY1NDAzOA
If it's a core Duo & not a Core2Duo, then it'll only run in 32 bit mode.
Lion/101.7 System requirements
• x86-64 processor (Macs with an Intel Core 2 Duo, Intel Core i3, Intel Core i5, Intel Core i7, or Xeon processor.)
• At least 2GB of memory, I say 6 GB
• Latest version of Mac OS X Snow Leopard (10.6.8), with the Mac App Store installed
• At least 4GB of disk space for downloading, I say 50 GB.
Like Snow Leopard, Lion does not support PowerPC-based Macs (e.g., Power Macs, PowerBooks, iBooks, iMacs (G3-G5), eMacs).
Lion also does not support 32-bit Intel Core Duo or Core Solo based Macs. Rosetta is no longer available in Lion, which means Lion no longer supports PowerPC applications.
http://en.wikipedia.org/wiki/Mac_OS_X_Lion#System_requirements
http://www.apple.com/macosx/how-to-buy/
What applications are not compatible with Mac OS X 10.7 "Lion"?
http://ow.ly/5Iz09
http://roaringapps.com/apps:table
