This site gives a few useful tips to consider before upgrading to Lion (10.7).
It is important to note that PPC apps will not work under Lion, as the Rosetta code is no longer included and is not even available as an option. For newcomers to Mac OS X this will probably pose no problem, but if you have older apps that are no longer being developed, they may be only PPC.
You will need to check this before you upgrade to Lion.
A quick way to find out is to highlight and copy the following line into Terminal:
system_profiler SPApplicationsDataType > ~/Desktop/MyFileTypes.txt
then find the resultant text file on the Desktop called:
in that file, search (command F) for this:
and if anything is returned, you have some PPC only apps. These will not work under Lion, so you would need to get upgrades, if there are any, or find alternative apps to do the job.