Update to 10.6.8 using the combo installer; http://support.apple.com/kb/DL1399
Snow Leopard 10.6 system requirements:
Mac computer with an Intel processor
1GB RAM (2GB or more preferred)
5GB available disc space
DVD drive for installation.
Lion is still only available from Phone Sales as far as I'm aware;
n the US, call 1-800-MY-APPLE and ask for a sales assistant; they'll give you a download code to use in the Mac App Store.
If your Mac meets the requirements you can go straight to Mountain Lion from 10.6.8 using the App Store.
Lion 10.7 system requirements:
Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7 or Xeon processer
2GB RAM (4GB minimum recommended)
OS 10.6.6 or later (10.6.8 recommended)
7GB of available HD space
Mountain Lion system requirements;
Your Mac must be one of the following models:
iMac (Mid 2007 or newer)
MacBook (Late 2008 Aluminium, 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)
And meet the same RAM and disc space requirements as Lion.