2GB >> Lower timings. That much difference (3-4-4-8 vs. 3-3-2-8) will not be noticable in anything other than synthetic benchmarks, and even then it will barely make any difference. 2GB will be a huge improvement over 512MB, it will blow your mind.
If your MoBo supports dual channel then just getting 1 stick is not going to gain you much.
If it does support dual channel then take advantage of it with a matched pair set and get the better performance.