I Have Thermaltake Hardcano II and one sensor I put on memory (33 up to 40 Celsius) and another one on the back of my Radeon 9000 card where the GPU is on the other side (with Zalman HP-80A - around 40 Celsius in use).
CPU and HDD have their own temp measurment (CPU - 38 C under load with Thermaltake Aquarius 2 and HDD 31 C).
My system Specs:
Sapphire Radeon 9000 64 DDR
Athlon XP 1700+
Biostar M7VIP motherboard
Seagate 40 GB HDD
2x 256 DDR RAM