U see,at low resolutions the gpu does not do all the work,the cpu has to also do some graphicwork as well as general processing,i have the same gtx 460 oc'es coupled with a i7 920 oced,im getting the same frames as you in the resolution 1600*900. so the culprit is your cpu,which cant keep up the work of handling/utilizing fully your 460.It would be good if you upgrade to a i3 2100 of 11k,a good motherboard of under 10k