I've got a Dell Dimension E521 (tower) where the PSU fan has started making strange noises, so I need to replace the PSU. Can I put any PSU in there or does it need to be a Dell part? If possible, please recommend a make and model that I can find easily in the UK...
From my experience with Dell machines, there has often been a slight size difference between a standard ATX PSU and the PSUs that ship with Dell machines. However, I have never actually failed to install a PSU in said machine, it was just usually missing one or two screws in a 5-6 screw attachment system, so I'd say yes, you can put a standard ATX PSU in said machine.
I can't really recommend any particular PSU models, but a Dell PSU replacement will generally be around $50, and a cheap PSU should be in the $20 range. You can get PSUs in the $10 range, but they tend to be unreliable, and just make more problems for you later.