What do I have to consider before upgrading my processor?

Well, you need to check which "socket" your current motherboard is made to accept, as the chip you want to install is socket (775) So you need to see if your motherboard will accept a socket 775 chip in the first place. Also some concern might be the power supply if it is a really weak 300W or something like that it might not be big enough especially if you run any sort of decent video card.