As you plan to upgrade to Lion, you must check to see if your Mac meets the following system requirements:
1) An Intel Core 2 Duo Processor or Higher (all 64 bit processors)
2) 2 GB RAM
3) 7 GB of free disk space
4) Mac OS X Snow Leopard 10.6.6 or higher (10.6.8 preferred)
5) Some services require an Apple ID
You can check on your system by accessing About this Mac under the Apple menu.