Those I7-920 are quite capable... with the right videocard. I wouldn't go getting rid of them just yet. I sold mine only because somebody was 'chasing' me to sell my computer. The replacement I7-3820 (o/ced to 4GHz) though a good solid CPU, doesn't render rF2 very much faster. I'm not complaining mind you. Both get the job done...well. I get around 80-114 fps with everything maxed out with the 3820. I got around 85-95 fps with the I7-920 overclocked to 3.7GHz again with everything maxed. The 920 can do the job with proper cooling. I used a CM Hyper212+ and a 2GB version of the 560Ti along with 12GB DDR3 1333 to get those numbers. Nothing special. The videocard, cooling and ram seem to make the biggest difference.
@OP. Cpu power is the key. Get rid of the standard cpu cooler and replace it with a better one. After that slowly figure out what the max ghz is for your cpu. I've got my phenom x4 running on 4 ghz for months now. Never a problem occured. Nice idle and max temps between 40 and 55c. Their are guys who are clocking it on 4,3ghz stable.
