QTP 8.0 is backwards compatible, and should run scripts developed in 6.5 without problem. The reverse is not true. Once a script is saved in 8.0 format, it can't be executed in 6.5.
Considering all that, I would strongly suggest you do a search on this forum for this topic. I've seen several posts a while ago dealing with the upgrade. Other users have sworn to have issues with the playback of 6.x scripts in 8.0. Whatever you do, make sure you have a backup plan.
I think you might be better off waiting for version 9.x of QuickTest Pro as there is some functionality from 6.5 being put back into the latest version. Log on to the Mercury website discussion forum and chech out the 'What's new for 2005' thread.