If this is your machine:
it appears that Lion (10.7.x) is the highest you can go. Mountain Lion (10.8) is a much better OS in my humble opinion. In any case, which OS are you running at the moment? Snow Leopard (10.6 and the OS just before Lion) is an excellent OS and will run some older applications which Lion and later will not.
You'd have to get both Snow Leopard (a disk) and Lion (a redemption code for download) from Apple.
Thanks babow, this helps a lot. The OS I have at the moment is Tiger (version 10.4.11), so quite old. Its just that I cant get a lot of updates for things like I-tunes, firefox, flash player etc as they dont seem to support the OS that I have. Will probably have a look at getting Snow Lepoard. What is wrong with Lion?
Nothing major as such except that it underwent major changes in the user interface and, more importantly, which apps you can use. If you're on Tiger, it means that you are running PPC apps which can still be used in Snow leopard, but not in Lion or beyond. So, depending on which ones you have and wish to keep using, you'd have to buy upgrades. Lion also made some major changes which some did not like and Apple reversed some of them in Mountain Lion, so to me, ML is what Lion should have been. Although you won't be up to date, Snow Leopard is an excellent and smooth running OS and can live on less RAM than Lion - it will perform well with 2 GB and snappier with 4, whereas with Lion and beyond, 4 GB can be considered the absolute minimum - more is better.
You can order Snow Leopard here:
That retail copy will not include iLife, so if you have that, don't delete it if you want it.