How do i secure my data when trashing a PC?

I already took out the HDD and am using it on new PC, but is there any other part i need to worry about?

The old PC crapped out and wouldn't turn on. i'm thinking a fried motherboard because the HDD and powersupply are working when i tested them.

I scheduled a pickup for it to be disposed of properly, i just want to be sure that no one gets my data and stuff. Anything useful i should keep from it? I assume the RAM, although it's not compatible with my new PC