if i take my iMac in for repairs and there is stuff on my desktop like a list of my bank passwords or um, personal pics of my wife, is my data safe from prying eyes?
If it's a 2009 27 inch iMac then it should be easy enough to remove the hard drive, especially if you've already opened it up to replace the power supply.
Have you got a backup? May change next steps. For future, as well as file vault you could use a password manager to add a second layer of security to data like that.