im actually stuck in the same boat with my macbook pro. we have leopard which they do not update anymore. Update to lion is 30 bucks but the problem is in order to have the app store to buy lion you need snow leopard installed onto the computer which is another 30 bucks. the lion wont install untill the software is at least 10.6.8 or something like that. There are a couple methods to trick the lion os into thinking your computer has snow leopard so you can skip that step and not have to pay for it… its on my agenda of things to do over winter break because leopard is ancient now.