you have Snow Leopard, right?
"Your Mac must have an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor to run Lion. Find out if your current Mac has one of these processors by clicking the Apple icon at the top left of your screen, then choosing About This Mac."
I think you're going to have to miss out on Lion, Macs are pretty particular about hardware, and everything else for that matter