I'd skip the video card upgrade altogether and just go with a Android device like a UG802, UG007 or an MK808 They all plug directly into an HDMI port and are compatible with Google Play...I have the UG802 but I'd recommend the MK808 because it has a better wifi antenna...
They can all do 1080p and are all wireless...Just avoid the MK802 its a single core A8...
The ones I listed are dual core Cortex A9 setups and are plenty fast...
and yes you can stream video over your network from a Windows or MAC computer to these devices....
You can find reviews of these all over the place...such as on youtube
OH and that Craig smart tv hdmi adapter cvd601 is a rebranded MK802....works alright @ 720p but why pay more for less...
You will find the MK802, MK808, UG007 and UG802 rebranded under several names...Its always cheaper to buy them under the model number rather than a random name...
And yes to answer your question these mini Android PC are worth it and they are AWESOME!!!
I seriously enjoy my UG802 but they do have limitations...
Here in the US you cannot use them on sites like HULU, CBS and NBC DUE to a lack of a DRM on them....you can however pay to have a DRM service put on those devices and them watch those services thru the DRM product...I however will not support that nor will I tell you the name of that DRM service because I believe its utter bullshit