Ok, problem solved...or better said "updated partially".
Here´s what I did (might be of interest for others running into that problem) :
1. Backup the old 10.4. partition with Carbon Copy Cloner onto a FW drive.
2. Install a clean Leopard 10.5.6 onto that partition
3. Install PT 7.4.2 (which works with Waves7 !)
4. Install Waves7
5. Reconfigure some system preferences - Start ProTools and GOOD !
Now I can easely jump from PT7.4.2 to PT8 and vice versa, and its fun.
Everything seems to run smoothly...im happy again
Intersound