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http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32899-7.html
That suggests that the R7 240 is between the GT 730 and GT 740. I would not look at anything below a GT 750, and GT 760+ if possible.
That GT 720 is regarded as sub-entry-level, and not much use for games.

I heard some time ago that AMD write in support for the ATI cards, and nVidia work better with Intel CPUs. So, If you have an AMD chip, go ATI.