Please check the list of tested graphics adapter for Photoshop CC 2015.
Also, make sure it meets the minimum graphics processor.
Hope this helps.
No, you don't necessarily have to replace your graphics card to use Lightroom CC and switching to Windows 10 shouldn't require a new graphics card anyway.
If you are experiencing slow performance, it might help if you describe exactly what is slow in Lightroom before you go ahead and buy a graphics card. Some situations of slow performance in Lightroom will not be helped by a faster/better graphics card. In particular, are you doing a lot of brushing or spot healing on your photos?
I’d get an nVidia (not off-brand) GeForce 6xx-7xx series with 1-2GB of memory, the newer the better. I have a 2-year-old 2GB GeForce GTX 645 and sometimes wish it was a little faster in PS and LR.
AMD/ATI (the other main provider of video cards) drivers don’t work with LR currently although I’m sure there’ll be an update soon.