In my opinion you would do the following: 1) Add memory 2) Install a large SSD 3) Don't worry about a video card.
I just installed a 500G SSD in my laptop and everything runs faster. That may be enough.
If you are not a power user of PowerDVD 12, what are you expecting to get from a video card? As Carl says, you might get a little bit of speed but you may also NOT get as clear a video. Is you video vhips up to date with the latest drivers?
Are you having issues with the program creating the shadow files (decoding)?
One of the questions I would like to have answered is how long does it take you to encode a video? I may be as short as 1:1 or as long as 10 to 1. What formats are you encoding/decoding? What is the output from the program - DVD, video files, etc?
There are too many variables to give you a definitive answer BUT as far a an external video card, spend you money elsewhere.
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BoilerPlate: To posters who ask for help -- it is nice to thank the volunteers who try to answer your questions !
Anything I post unless stated with a reference is my personal opinion.