I have not had a lot of issues with processors (am successfully running Feisty on a P500 without issues), but memory is important.  You don't actually give a specification, but I'd say that 256MB was a minimum for Gutsy and probably 512MB would be preferable as it will give the OS room to breathe... if you are only on 128MB or less then I'd upgrade the memory before I upgraded the OS.  Obviously you also need some hard drive space... a 10GB partition would probably be the minimum. Sean