Steam – How to make sure a GPU will support most games

You should run into no problems. If you're hurting for disk space, you could delete it, but your OS knows which one is which and they are not sharing resources.

Unfortunately there isn't really a concrete way to look at models numbers and know how they will perform or which is better. Model names are arbitrary and naming schemes may change from time to time. However, manufacturers have some incentive to have at least some kind of coherent naming scheme at any given time for marketing purposes. If you want to compare specifications of a particular card (for example: your own graphics card) vs the required or recommended specification you can look up both at the manufacturer's website and attempt to make an informed decision. Also note that 3rd party services and benchmarks are usually available that will give you a moderately accurate numeric representation of the graphic cards' performance. A third alternative would be to search the internet and see if anyone is running that particular game on either the same or similar hardware.

Generally, you can check either the manufacturer's website or a retailer to try and glean information on the naming scheme. In the Nvidia example GTX 650 vs a GeForce 840M. You'll probably be able to note that the "naked" GeForce branding in general isn't as powerful as "GeForce GTX" and that the "M" stands for mobile and will generally be lower performance and feature limited but also require less power. To go into specifics the first number is (for example: the 6 in 650) generally denotes the generation of card and the following numbers will normally donate the intended performance.

So to give some full examples, a GTX650 compared to a GTX750 will likely be very similar in terms of performance. The 750 may run cooler, require less power or have additional features but any difference in actual performance will be at best marginal assuming that the manufacturer didn't change their naming scheme. Another example I can give that will be more illustrative would be a GTX 590 vs a GTX 960. The 960 is much newer, has more features and so on but isn't close to the performance of the 590 (despite being a much bigger number!). However, it's worth noting that with such a large timeframe between the two cards, the gap between the performance of an old "X90" and a new "X60" will slowly close due to improvements in technology, manufacturing, and to a minor extent developers targeting and testing with newer graphics cards.

As I said earlier, it's mostly arbitrary and Nvidia have changed their naming scheme between the 7600 GT and the GTX 650. AMD also have their own naming schemes that they can change too (and they are likely to differ from Nvidia's at any given time). Your best chance at a general rule is to check a 3rd party website and hope that they are accurate. The example given in your post could be checked on a website like this.

Developer hardware recommendations are sometimes inaccurate. However, from that information it looks like you should be able to play it on lower settings but YMMV.