Dell OptiPlex 755 DT upgrade

Dear All,

A few years ago I bought a second hand Dell OptiPlex 755 DT form factor desktop PC from a local PC shop.  If I'm correct this PC comes with a 280w PSU and an onboard Intel Q35 GPU.  The PC has a free PCI Express x16 connector.  I want to upgrade my VGA.  Due to the PSU the choice is rather limited.  Via this forum I'll hope to find the necessary answers to my questions.  Can the PSU be upgrade to a 300w or higher PSU taking into account that it is a DT form factor?  Can this be any PSU or do I need a specific one from Dell?  I'm pretty enthusiastic about the Asus NVIDIA GeForce GT 610 SILENT 2GB DDR3 but this card requires at least a 300w PSU hence my question.  Without the PSU upgrade I was recommended this card by MyCom : Asus radeon hd5450 silent,1gb.  Any other suggestions regarding the upgrade of my VGA card with or without a possible PSU upgrade?  Can someone please help me out with my questions? 

Thanks in advance.

Kind regards.