Yeah you'r right. same with me i've XFX GT240 1gb. its because games don't need just Good Gpu. A game want. Combination of Good Processor+More Ram+Good Gpu.
you'r ram and gpu is good. but your processor is not so good. thats why its unable to handle that game on more FPS. but your Gpu is good thats why no matter what is resolution it can play it on any resolution easily