cooling and OC question plz help

I've recently been trying to OC my comp and I've been following all sorts of guides to no avail. Every time I try to OC, its makes my PC run worse.. I think it is because my rig is overheating... in my bios it says that my system temp is 28C but my core temp is 80C! My first question is,
1.for my the gigabyte S3 is there a program to tell me the temp. besides looking at the bios? I've been looking for various fans/heatsinks and I think I want the Thermaltake BigTyphoon
(http://www.newegg.com/Product/Product.aspx?Item=N82E168...) but
2.I want to know if you guys think it will fit in my case?
(http://www.newegg.com/product/product.asp?item=N82E1681...)

E6300
BFG 7600GT
GIGABYTE GA-965P-S3 LGA 775
Rosewill R804BK Black Steel ATX Mid Tower
LOGISYS Computer PS575XBK ATX12V 575W Power Supply
pqi TURBO 1GB 240-Pin DDR2 SDRAM DDR2 667 (PC2 5400)