Thanks for the help guys. I'll take it out again and have another look.
It wouldn't bother me so much but I'm waiting for my mobo for my new rig (have every other component over a week now

) and have to use this one. The sound off the PSU fan is unbelievably annoying.
edit - Just been told that the Dimension 4300 is standard ATX but the case isn't so I'm going to have to cut a bit out of the back of the case to get the new PSU in. Ah well, at least it can be easily replaced.