I've done it! I hunted around and got in touch with a few companies about replacing the power supply and ended up buying a power supply from a company called PC Power who were able to confirm the suitability after I raised a support ticket with them. The PSU I purchased is http://www.pcpower.com/products/description/Silencer_500_Dell/index.html
I installed the psu in my Optiplex 760 mini tower last week and the PC has been working quite happily. It is very quiet and the only caveat I would add is that in a mini tower case there are a lot of cables to manage from the Power Supply as it provides a lot more options than the standard PSU.
Very happy with the machine - running a more powerful nvidia card.