It currently has a 300 watt psu installed and the graphics card requires a minimum of 400 watts and i cant seem to find a good graphics card that can be ran with 300 watts much less a low profile one.
The link that you provided for your system shows that you have the mini-tower version.
That version is able to handle full height PCI-E x16 graphics cards with a length of up to around 9.5 inches before the hard disk drive cage interferes.
If you owned the slimline version (i.e. Inspiron 660s) then you would be limited to low profile graphics cards.