Get a single bigger SSD, windows takes up <15GB and smaller SSDs are slower and have lower life span (essentially less spare memory). Buying SSD soley for games is something you'll do if you have nothing else you could possibly buy because it only effects loading times and you can fit several games + OS + programs onto a single 256gb anyway.
840 is ok (especially as it's pretty cheap) but it is a budget SSD so there are faster ones out there that would last longer (the nand used in the basic 840 doesn't last as long as stuff used in most SSDs, not that it's an issue with 256GB as it'll probably fail or be incredibly outdated in 7~10+ years it'll take to wear it out). 830 is slightly faster I think, 840 Pro is much faster/better but a lot more expensive, Corsair Neutron series is meant to be pretty good as well (using a new controller).
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