Me'thinks the 7900GTX would be better due to the clocks being higher than the 7950...
Video In Video Out, usually seen as the acronym VIVO, enables video cards to have bidirectional (input and output) video transfer through one connector, by using a specialised splitter cable. VIVO is commonly pronounced "Vee-Voe", but other pronunciations are in use.
VIVO is featured on some video cards, such as the Nvidia GeForce range and the ATI Radeon series. It can receive input from RCA or S-Video sources. However, at this time, S-Video is more widely used for VIVO by both Nvidia and ATI. Basically what this does is allow you, for example, to watch TV on your computer, and then without switching cables, watch what you would see on your computer - on your TV.
Some VIVO cables also support the output of component luminance, and 2x chrominance, enabling HDTV support. However as this connection is not digital it does not support HDCP which would be required for official HDTV support as set out by the EICTA.