At stock, the GT740 is as fast as a GTX260 or HD4870 but the DDR3 version has only 1/4 the memory bandwidth of those.
It should be fast enough as-is to play older games at 768p and overclocking the core won't really change this at all because the bottleneck is the memory. If you want to try overclocking it would be advisable to focus on getting the memory speed as fast as possible, although note it will never match even half the GDDR5 version's bandwidth.
You can use any 3rd party overclocking tool to set and test overclocks, and when you are satisfied with the settings, can lock it into the BIOS with KeplerBiosTweaker.