it depends a bit on the kopete version and the protocol. the latest kopete version in kDE 3.5.6 can transfer files with almost every protocol, but previous versions only where able to use certain protocols. trying another messenger is indeed a wise choice here... hopefully, the new 'Decibel' framework in KDE 4 will fix this, as there will be a standard library for communication, shared between chatclients like gaim and kopete.