All the suggested cards are in the same ballpark as the HD 4600, it would be wasting cash on less/=/marginal performance difference to buy any of them.
A GT730 GDDR5 might offer an upgrade you'd notice but a recent ~65Watt TDP card like the GT740 GDDR5 would be a better upgrade. A single 6-pin PSU connector might be needed for one of these - check the maker's site and, as Bill said, your own PSU, before buying. If you intend to play games, your monitor's resolution is very important - the higher the pixel count, the better the graphics card will need to be.
Don't fall into the trap of thinking that more is better when it comes to graphics RAM - 1GB of GDDR5 is way faster than 2GB of DDR3 :)