If you are not going to OC,then timings aren't important,generally timings/voltage/Mhz of the RAM are mainly important for OC'ng and won't have a great impact on performance however choosing 4GB is the better choice because it overall performs better than 2GB.
As a rule, faster ram or better timings show significant performance gains only in synthetic benchmarks. For real applications, or an increase in FPS it does not scale well. To my mind, 4gb is very much better.
Memory quantity always trumps memory speed/timings when it comes to gaming. Waste your money on a better videocard, if you want to see a real improvement.