Lion 10.7 System Requirements
• Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor (most models released since late 2006 - early Intel-based Macs with Core Solo or Core Duo processors aren't compatible)
• 2GB of memory (recommend at least 4GB -- and ideally even more, as you'll see benefits in many computing tasks)
• OS X v10.6.6 or later (v10.6.8 recommended) for Lion download from Apple App Store; or Leopard 10.5.x if using Lion on USB drive
• 7GB of available space (10GB recommended)
If your Mac processor doesn't meet the requirements or you have less than 2GB RAM, you can't install Lion.
Cheers, Tom 😉