NEED HELP ON POWER SUPPLY

Pease help..ive looked everywhere online for a solution, i have a dell 380 low profile desktop with only a 235w power supply, ive upgraded to windows 7 pro 64 bit from a 32 bit and upgraded to 8gb ram, i want to lightly edit video with a better video card-- Can i use an after market psu with at least 300w/305w ? i can customize demensions no problem but im being told i have to use a dell psu,but my dell was built in 2009 should it matter?