I suffered with over heating for aabout a year on a previous pc and tried all sorts of solutions to cure the problem.
First off 70 degrees at rendering is not great but not unusual with a P4 which run hotter than Athlon in general(65 -70 would be about normal for 100% cpu usage).If you pc does not overheat and shut down like marlon said his does then it can probaly take it although anything you can do to decrease the temp would be good.
During my overheating period I changed my case twice,added casefans galore(12 in the end),my cpu fan a couple of times once to water cooled wich actually made no difference and my motherboard also.
The final solution was to buy a sever tower case (mine was and is a Coolermaster cmstacker) with loads of airflow room and the thing that made all the difference in the world a crossflow fan that blows air across the motherboard wich in turn helps the cpu fan with cool air.
My cpu temp almost halved with this solution.