Im currently using the stock Dell Dimenion 4700 power suppy with Depth: 4"; Height 3.25"; Width x 5.75" as its dimesnions, im looking to upgrade the power supply to allow me to upgrade other components. If i bough a new power supply do the dimensions have to be exactly the same or do i have some space to adjust?
The Dell Dimension 4700 doesn't require a proprietary power supply, so you should be able to use any standard ATX unit with either a 24-pin, or 20+4-pin motherboard connector and at least 2-SATA power connectors.
To be sure you can compare the dimensions of the unit you would like to purchase against the space available for the power supply within the Dell Dimension 4700's case.
The GeForce 8400 GS Graphics Card does not support SLI so you won't gain any performance improvement using multiple instances of that card. You need an SLI capable graphics card if you're sticking with an nVidia GPU.
A minimum of a 350 Watt Power Supply is what is recommended when using one of the graphics cards that you have installed.