Dell Dimension 4700 CPU upgrade

Hi, I have a dell dimension 4700 and im looking to upgrade my cpu. Inside currently is the pentium 4 530 (LGA 775). Im looking to get the 550 which is rated at 3.4ghz but could I go higher and would this cause heat issues? And one last thing the fan in the green shroud at the back of the case is blowing in, is this normal as my case is warm to the touch?