That OEM case either Dell, Hp, Compaq is limiting the air intoflow inside the case causing stagnent warm air. My thinking is that the bracket on the back of the mobo isn't correctly installed limiting the pressure being applied to the heatsink. The only way you would get higher temps is by poor cpu contact or bad air air flow. I would buy a cheap aftermarket case that allows better cable management and increasing air flow to your components. The artic cooler you have on your gpu is just dumping hot air into your case and with nowhere to go it gets pulled through the cpu heatsink causing higher temps. antec 300 is a good cheap case that has plenty of room and air flow. good luck.
