most video cards will suite you fine for PS. Most of the video cards now stress 3D speeds which are not important for PS. Anything like a R9700 is complete overkill. A $50 video card will hardly be different.
99% of the times PS's bottleneck doesn't come the video, it comes from the CPU , RAM and HD not being able to feed to the video fast enough and therefore you may get undrawn screens.
and..
Scratch space is mainly temporary work/swap space for the application to store extra versions of the file you are working on. When you press undo, that file is being quickly accessed from scratch sapce so it doesn't have to reprocess the changes again. If you notice that applying a filter may take 30 seconds, but hitting UNDO will only take a second. You can also command most graphical design programs to use a certain amount of scratch space or remember a certain amount of undo's. The older days when RAM was in small quantity, swap space was done on the hard drives where drive sped was very important. Now with machines with massive amounts of RAM, its really not as important how the scratch sapce is setup on a HD. All my machines running PS have 1gb of RAM yet they probably never take more then 200-300mb of usage with my 20-60mb files. But then I like to have other graphics programs like Quark and Illustrator on as well like most designers.