Are you using the sandybridge/ivy i5? If so, I don't think you even need a gpu as you should be able to use the onboard gpu.
As for multitasking, yeah it could help but it depends on what kind of tasks you want to do while watching videos. Obviously gpu heavy tasks would benefit the most (video editing, encoding,photoshop).
You could build the i5 machine first and then see how it performs with the tasks you want, if it's not fast or smooth enough, you could buy the video card later.
For SBS and TnB you play it on the pc and set SBS on the TV, the TV does the processing not the PC.