1st you want to verify or Repair if needed the Hard Drive, not Permissions.
A complete clone backup would be best in case there's any problem, or you find out 10.5 was better than 10.6 or 10.7 for your use.
Then make sure you plenty of free space & the other requirements for the steps, which are buy 10.6, install 10.6, update 10.6 to 10.6.8, then you can buy the download only 10.7/Lion.
Snow Leopard/10.6.x Requirements...
* Mac computer with an Intel processor
* 1GB of memory (I say 4GB at least, more if you can afford it)
* 5GB of available disk space (I say 30GB at least)
* DVD drive for installation
* Some features require a compatible Internet service provider; fees may apply.
* Some features require Apple’s MobileMe service; fees and terms apply.
Which apps work with Mac OS X 10.6?...
It looks like they might still have it...
If it's a core Duo & not a Core2Duo, then it'll only run in 32 bit mode.
Lion/101.7 System requirements
• x86-64 processor (Macs with an Intel Core 2 Duo, Intel Core i3, Intel Core i5, Intel Core i7, or Xeon processor.)
• At least 2GB of memory, I say 6 GB
• Latest version of Mac OS X Snow Leopard (10.6.8), with the Mac App Store installed
• At least 4GB of disk space for downloading, I say 50 GB.
Like Snow Leopard, Lion does not support PowerPC-based Macs (e.g., Power Macs, PowerBooks, iBooks, iMacs (G3-G5), eMacs).
Lion also does not support 32-bit Intel Core Duo or Core Solo based Macs. Rosetta is no longer available in Lion, which means Lion no longer supports PowerPC applications.
What applications are not compatible with Mac OS X 10.7 "Lion"?