If there are spacers with the case use them. Some motherboards need them to be positioned correctly for the back plate access to be right (the video and sound cards you plug in etc)
The spacers also let more air circulate around the back of the motherboard.
Precautions to take!? Watch out for static, make sure you ground yourself before touching any components and try to touch them by the plastic bits. Dont get greasy fingerprints over them, especially the CPU and HSF.
Be gentle and careful, brute force will not achieve much.
Read the manual!
Your PSU is only 450W so dont be sticking in a power hungry graphics card.