The detailed specs tab of this card says it requires a minimum of a 400 watt power supply. You might need a card that consumes/needs less power.
As I understand it, PSU power gets eaten up by other things in your PC, too, like your hard drive and CPU, etc. So you probably don't want a card that uses too much more power than your current one, as 300/305 watts isn't much to share amongst your stuff, unfortunately.
Also, your PC looks to have PCI Express x16, whereas the card you linked to is PCI Express 2.1 x16. I read that 2.1 is backwards compatible okay, but the performance you get may not be as good as you expect.
Have you thought of increasing your RAM instead of graphics card? Sometimes that can bring nice speed improvement to hold you off from an upgrade for a while. Looks like you have four total RAM slots available to work with. Have you already maxed them out? I think sometimes a motherboard BiOS flash can even increase the amount of RAM memory it can support, if the manufacturer has such a BIOS update available for your motherboard; unless I'm confusing this with hard drives. (I've upped the hard drives my mobo can support by flashing the BIOS before. Can't remember offhand now if RAM can work similarly.)
If you haven't seen, here is what appears to be your manual, showing various specs: http://support.dell.com/support/edocs/systems/dimE520/en/OM_en/NH730A01.pdf