There are two different Dell Inspiron 620 models. The Inspiron 620 and 620s desktops.
The Inspiron 620 has a Mini Tower Micro ATX form factor and a standard ATX12V 300 Watt Power Supply.
The Inspiron 620s has a Slim Tower and a 250 Watt TFX12V Power Supply.
I've seen an Antec NEO ECO 620W PSU installed in the Inspiron 620 Mini Tower with no problem.
Are you planning a graphics card upgrade? If you are, please specify the graphics card that you are planning to use. This information is needed to determine the appropriate power supply capacity needed to properly power the system.