Memory runs as fast as the slowest stick, so if you use 1333 with 1600 then the 1600 stick will run at 1333. I think the 460 2gb plus making sure you have at least 8gb ram will give you the best game performance you can have with that computer, at a reasonable cost, for those games.
If you need storage space as a priority, such as for a large game library, then a hard drive is a good investment. If you only play those games then I'd suggest getting a 120gb SSD for your OS and one or two games, and using your current hard drive as the main storage for the other stuff. The SSD will not help game framerate, don't get me wrong, but nothing perks up an older system like an SSD. Everything will just feel more responsive, usability of the computer overall will be a lot better.