Question Compatibility of GT 730 graphics card with Dell 3020 MT Desktop PC ?

I am using the original PSU which came with this computer.
You might want to source an aftermarket PSU that's reliably built and has 450W of power at the entire system's disposal+an adaper to adapt an off the shelf PSU to work with the proprietary connector on the motherboard;

As for the GPU dropping into the case, yes it'll fit, I'd just get a PSU with the GPU's purchase to wrap up the upgrade.
 
This gt730 Graphic card uses 300 watts of power whereas my PC Power Supply is 290W. The shopkeeper from where I bough this card told me that it will work without the need to upgrade the Power Supply. I am not an ardent gamer and might use this GPU for some light graphic programs. I would like to know if it will cause problems for my PC by using the existing power supply.