If your not gonna overclock, then you have nothing to worry about - Those temps will not have a performance hit, just gives you no headroom for overclocking. If you wish to do so then investing in a good heatsink is pretty much a given regardless of temperatures. If your not gonna overclock it would be a waste of money to get a new heatsink/cooling unless you want a quieter system, i've ran orthos on my comp for 24 hours at cpu temps over 68C~ constant and it was fine.
My rig idles at 46C and thats at stock volts, also taken out the floppy drives and one of the cd drives and put a 120mm fan over it. Makes a hell of a lot of difference at load temperatures, It would be worth attempting a mod such as ging's or a side blowhole as the system temps inside Sonata's get toasty - before i put my intake fan in my system, my system temp would be about 40C with cpu at about 66C full load now its 57C full load and system temps around 30C and thats with all my fans at min rpm.
As for the temperature issue again, your temperatures seem fine for the cooling capacity you have and your system temperatures are pretty toasty. So i think investing in a new cpu cooler really would not effect your temperatures much, i know mines did not but putting in a intake fan that takes air from directly out of the case into it made much more of a difference but saying that the Arctic Pro is so cheap theres no reason not to get one.