I'm not an nVidia user but have you loaded their latest drivers -- or a set that folks find works well for them which often is NOT the latest. Upgrading from one AVR to another shouldn't require you any change to your HDMI cable unless you're trying to do 3D or something that required the 1.4 standard, or are you?
Your new AVR may be trying to assess the video signal, upscaling it, or processing it somehow before it can pass it onto your display... have you checked those settings?
As far as changing to another video card you can't beat the bang for the buck with any of the AMD: HD4550, HD5450 or HD6450