Have you checked the games' developer's forums to see if there is an issue with the current driver and that game? WoW as you said should be able to run on lower end systems, so your system should be laughing at it at this point.
When you say stereo feedback, are you talking about audio or video? If it's audio, does your monitor have built in speakers? If so, the HDMI has the ability to send the audio signal and video signal in one cable (at least it definitely does for LCD/Plasma TV's). THe monitor may be getting the audio signal from the cable, and if you have a mic plugged in that is too close to the speakers, that would cause feedback. When you installed the 260, did the drivers also install an audio codec/driver?
Temps, yeah I would check that contact between the proc and the HS. It's a longshot, but that *could* also be the cause of some of your game stuttering....probably not though since it is only in a select few games.