No performance hit to speak of really, i will get very brief hiccups at some very texture intensive scenes.. such as the outskirts of whiterun in the farm areas. This is with the beth hd dlc plus a bunch of actual high res mods. I use Beths HD pack mostly for weapons/armor (which i think they did beautifully) and some of the architecture for cities and filling any gaps left from my mods. My system isnt that fancy either.. i3-2120 (2 cores with HT), 8gb RAM, 6870 1GB. Since the 1.4 patch my FPS has been great, no noticeable slow downs. One thing you might try is using FXAA instead of MS or SSAA. If you get the post process injector you can sharpen the FXAA effect to be the same as if you were using MS or SS (the antialiasing quality is obviously less but it will increase performance!) and if you dont care for the color stuff you can turn that off.