Typically the standard shipped heat sink and fan are the cheapest thing the manufacturer can put on there and keep the computer functional until your warrenty runs out. If you want to verify that it's not the heatsink, use a better heat sink. Case helps more with CPU temp if your running high heat from extra hard drives, beefy video cards, extra DVD, CDROM, and burners. If you only have the typical, 1 hard disk, 1 CD/DVD Drive, Floppy, 1 video card, 1 sound card, 1 network card setup, then you probably won't notice a case fan help much. But if you have say 2+ Hard drives, a CDROM AND a DVDROM/Burner, plus all the other stuff, and perhaps the room your in is over 75f degrees, then you may notice some help from a case fan.
Try either a new processor of the same type or a new CPU Heat sink. Also make sure your Heatsink has a fan to help with the heat disapation.