Overclocking is a way of making the processor run faster than it normally would. By doing so, it can make the processor do more work and thus appear more powerful. This is achieved by supplying the processor with a bit more electricity and requires (1) a processor capable of being overclocked - modern processors, especially from Intel, tend to label these as 'unlocked', (2) a motherboard which is capable of overclocking, and (3) a high quality power supply unit for the extra electricity.
GhislainG brings up overclocking because it can be a way of getting more from a processor when processor upgrade paths are exhausted.
Browsing around, there seems to be two variants of your PC: two different cases - a Small Form Factor and a Convertible MiniTower. You mentioned 'tower' in your first post (the Convertible MiniTower I assume), which should allow a bit more room for a more standard sized card. Even something like
http://www.evga.com/Products/Product.aspx?pn=02G-P4-3751-KR should fit into it. I am wary of the length, as it could be a tight squeeze.
A safer option would be something like this Gigabyte low profile version of the GTX 750 ti:
http://www.gigabyte.com/products/product-page.aspx?pid=5160#ov
Tried to find a low profile version of the RX 460 with no luck, and some threads seem to suggest the GTX 750 ti is the only option.
Check the dimensions of your case. The Convertible MiniTower should be able to accommodate most graphics cards (size wise) assuming they're not overly long (the problem I had, and even then it was the power cable more than anything else).