Sprogget,
Replacing a PSU is infact quite easy especially if you spend a little more and get a PSU with a modular cable system. You wont need all the cables probably and as you take one off the motherboard/device you place one of the cables from the modular system in its place. Once all are disconnected from the original PSU you can then just remove the PSU by undoing the retaining screws, attach the new one, plug the modular cables in and the job is complete.
One word of warning I have not seen mentioned is to make sure that your replacement PSU is at least as capable as the previous. i.e. dont replace a 500w PSU with a 450w PSU as it may not be able to cope with your system.
Cheers
Jof