I have an old MacBook, from 2006 and currently have Snow Leopard installed (after purchasing it in 2010), I want to upgrade to Mountain Lion mainly because I want the Xcode 4 which looks 100% better than the current Xcode I have. However, when I try to buy the OS X Mountain Lion it tells me I do not have the correct processor. After a little research I saw an article that said some OS X Lion users could not upgrade to Mountain Lion because Mac is no longer supporting its older models like MacBooks before 2008 or something like that.
So this lead me to the idea maybe I don't have to upgrade to Mountain Lion, maybe I could upgrade to Lion and still be able to get Xcode 4. So, I tried to find a way to upgrade to OS X Lion, but I could only find the upgrade to Mountain Lion. Which is frustrating because I don't want to buy a brand new MacBook just to get Xcode 4, its nice, but not that nice. But Apple should allow me to at least upgrade to Lion, because they decided to no-longer support this older hardware I am still using.
Choose About this Mac from the Apple menu; Lion requires a Core 2 Duo(not Core Duo) or better CPU and at least 2GB of RAM. You don't need to buy a brand new Mac in any case.