Before you jump and buy the PSU. That PC needs an adapter because the 24 will not just fit. Dell has custom 8 pin connector.
Here is the adapter I found on ebay.
http://www.ebay.com/itm/24-to-8-Pin-ATX-Dell-Optiplex-3020-7020-Power-Supply-Motherboard-Adapter-Cable-/112054549450?hash=item1a16f8bbca:g:qBoAAOSwH71XPGT6
(It's in UK)