I haven't done a test for BF2. I have seen the graph draw for memory use, and it hit 0% on a 1gb system. For fear it uses exactly 1242mb's of ram. http://img.photobucket.com/albums/v5...memoryfear.jpg
Anyway, if overclocking you should go with the performance ram. baring in mind, that adding more, you will have to re do the overclock, and you are not as likley to be able to go as high.
What is the stock speed of the 640? If you could squeeze 2gb's of performance ram into the budget (2x 1gb sticks.) it would be better, or if it is only a small OC you could get away with the value.
With 1x 1gb you lose dual channel, so will not get as much bandwidth.